Swearing and cussing - is it a sin

Scripture cares intensely about speech, and it does draw lines - though they may sit differently than expected. What is unmistakably forbidden: using God's name emptily or as an exclamation ('Thou shalt not take the name of the LORD thy God in vain', Exodus 20:7) - that is the one category Scripture treats as grave without qualification. Beyond that, the standard is about what speech does: 'Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying' (Ephesians 4:29); 'let it not be once named among you...

neither filthiness, nor foolish talking, nor jesting, which are not convenient' (Ephesians 5:3-4); 'out of the abundance of the heart the mouth speaketh' (Matthew 12:34), and 'every idle word that men shall speak, they shall give account thereof' (Matthew 12:36). So the deeper issue than a banned-word list is trajectory and target: words used to demean people made in God's image are condemned even when technically clean (James 3:9-10 - blessing God and cursing men with the same mouth 'ought not so to be'; Matthew 5:22), while coarse filler betrays a heart worth examining rather than excusing. If cussing has become your reflex, do not just bite your tongue - ask what is filling the well the bucket keeps drawing from (Luke 6:45), and pray David's prayer: 'Set a watch, O LORD, before my mouth; keep the door of my lips' (Psalm 141:3).

Grace covers the slips; direction still matters (Colossians 4:6 - 'Let your speech be alway with grace, seasoned with salt').