As mentioned in a previous post, the Bible gives no list of words which are profanity.
Words transmit ideas; they are a means of communication. It is, therefore, the idea associated with an expression that can create an evil word pattern. As such, there are guidelines given in the scriptures about how a person is to use language. Usually, profanities are classified into three main areas: obscenity, vulgarity, and blasphemy.
The last, blasphemy, is not debatable, it is a sin as given by the third commandment in Exodus 20:7
“You shall not misuse the name of the LORD your God, for the LORD will not hold anyone guiltless who misuses his name.”
Anytime anyone misuses the name of God–connecting it to or using it as a profanity–it is a sin. The word in Hebrew to describe this misuse is שָׁוְא. It carries with it the idea of saying God’s name in a worthless way. This appears to be how most use God’s name when not connecting it to any religious meaning. As such, it is profane.
We can also profane God’s name by giving worship to something other than God, as seen in Leviticus 18:21
“Do not give any of your children to be sacrificed to Molech, for you must not profane the name of your God. I am the LORD.”
This is also a violation of the second commandment found in Exodus 7:4-6. There can be no argument and no reason that anyone at anytime should profane the name of God.
Moving back to the original question, “How does one make a decision on obscenity and vulgarity?” Again, the Bible has no list of “no-no words,” but it does give guidelines. In what follows, attention will be given to the pertinent scripture passages dealing with foul speech and Christian conduct. In this look at the Bible, only New Testament passages will be addressed.
The the seminal passage for this topic is Ephesians 4:29, which reads,
“Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen.”
Σαπρoς is the Greek word here for “unwholesome,” and means bad, decayed, or rotten, which is applied typically to decomposing vegetables or animal carcasses. It is also used to describe a tree which produces no good fruit in Matthew 8:17. This word also has a moral sense in which it describes something depraved, evil, contaminating, anything obscene, or offensive. 
Paul here forbids all kinds of “foul-mouthed” speaking. He doesn’t just stop with a negative command, but goes on to put the same thing is a positively. Christians are not only to avoid saying anything rotten, but are to use words that are helpful for building others up.
The Christian should be known for his pure speech–speech that is characterized by words of help to others.  It is extremely unlikely that foul speech would ever do anything to lift someone up, in fact this verse puts the two at odds with one another. A positive example of helpful speech can be seen in Job 3:4 which says, “Your words have supported those who stumbled.” These supporting words are the kind of words that Christians should be using. Barnes says,
“A Christian should be pure in his conversation. His master was pure. His God is pure. The heaven to which he goes is pure. The religion which he professes is pure. Never should he indulge himself in an obscene allusion; never should he tell anecdotes of an obscene character, or smile when one is told by others. Never should he indulge in a jest having a double meaning; never should he listen to a song of this character. If those with whom he associates have not sufficient respect for themselves and him to abstain from such corrupt and corrupting allusions, he should at once leave them.” 
From Barnes’ commentary it is clear that Ephesians 4:29 calls for nothing less than purity in speech and in all areas of life.
If Ephesians 4:29 was not clear enough Paul writes in Ephesians 5:4,
“Nor should there be obscenity, foolish talk or coarse joking, which are out of place, but rather thanksgiving.”
The context of this passage is Paul calling believers, in verse one, to be imitators of God. God would not participate in such shameful language and neither should believers. The word in this passage translated obscenity comes from the Greek αἰσχρότης which carries the idea of shameful or deformed. Foolish talk is the compound Greek word μωρολογία which conveys the idea of godless words. It seems as though Paul is encouraging believers to aim their conversations in a more edifying manner. 
For instance Paul writes in Colossians 4:6
“Let your conversation be always full of grace, seasoned with salt.”
Believers should consistently be trying to direct their conversations, from the obscenity and foolish talking, toward thoughts of God rather than godless words.
Paul again writes in Philippians 1:27 “Whatever happens, conduct yourselves in a manner worthy of the gospel of Christ.” The word for “conduct” comes from the Greek word πολιτεύομαι which means to live as a citizen. The implication is that believers conduct should distinguish them from the rest of the world, including speech.  Paul also pushes the Philippians to new heights of obedience to this “worthy manner” of living in Philippians 4:8
“Finally, brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things.”
Believers are not only to have worthy conduct but also worthy thoughts. Citizens of Heaven should not only be on their best behavior on the outside, but in their thought life as well.
In Colossians 3:8 Paul writes,
“But now you must rid yourselves of all such things as these: anger, rage, malice, slander, and filthy language from your lips.”
Interestingly, the last three vices in this verse are all forms of communication. Additionally, the first two can also be expressed in communication. The word for filthy language is αἰσχρολογία and means low and obscene speech. Barnes says that,
“The conversation of the heathen everywhere abounds with this. A pure method of conversation among men is the fruit of Christianity.” 
In this verse we can turn the negatives into positives and find three laws for Christian speech: (1) Christian Speech must be kind. (2) Christian speech must be pure. And (3) Christian speech must be true.  There has never been a time when so much filthy language is used. An informal poll a few years ago revealed that nearly 75% of the people had been exposed to profanity while in public!
Finally 2 Timothy 2:16 says,
“Avoid godless chatter, because those who indulge in it will become more and more ungodly.”
The term translated “godless chatter” is from the Greek βέβηλος which carries the idea of profane, common, or ungodly. This verse is most certainly speaking to the downward spiral that beginning to use profanity starts. I have seen this happen first hand. Someone starts to spend time with those who swear, then they use a few words themselves, before long they are using foul language in their common speech. The Bible says that those who use profanity eventually become more and more ungodly. This happens as they desensitize themselves to living a godly life and begin to adopt worldly practices.
The scriptures give a very clear guideline when it comes to profanity.
Check out the other posts here:
- Profanity, Introduction
- Profanity, What is it?
- Profanity, What does the Bible say?
- Profanity, Should Christians ever use it?
 Albert Barnes, Barnes’ notes on the New Testament (Grand Rapids, MI: Kregel Publications, 1982).
 William Barclay, The Letters to the Galatians and Ephesians: Revised Edition (Philadelphia: Westminster Press, 1976).