Mat 5:34 But I say unto you, Swear not at all; neither by heaven; for it is God's throne:
Mat 5:35 Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King.
Mat 5:36 Neither shalt thou swear by thy head, because thou canst not make one hair white or black.
Mat 5:37 But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
I suppose a person used to swear by things greater than himself to give the words more power and authority. Jesus said that we are not to do this. I think of swearing as the use of the Lord's name in vain, Such expressions degrade and blaspheme the holiness of God.
Lev 22:32 Neither shall ye profane my holy name; but I will be hallowed among the children of Israel: I am the LORD which hallow you,
Casual slang has become common place, even sometimes in a church setting popular expressions are used. Where do these expressions come from? Jesus says they are evil. Humor is often used to degrade someone or to make fun of someone, we must watch out for this too, for it's origins often are evil. Comedians in the 1940's were witty, but most of what is heard today is evil and degrading, not even funny.
Let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
1Ti 4:7 But refuse profane and old wives' fables, and exercise thyself rather unto godliness.
Does this sound boring to you? Well, how about speaking in psalms and proverbs or singing of hymns?
Eph 5:19 Speaking to yourselves in psalms and hymns and spiritual songs, singing and making melody in your heart to the Lord;
Speak the word of God and words that build up the hearer. Words of truth from the Holy Bible are powerful and constructive. People need truth and love. This is what God offers and we are to share these with others.
Eph 4:29 Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers.