Is it a sin to cuss / swear / curse?

It is definitely a sin to swear (curse, cuss, etc.). The Bible makes
this abundantly clear. Ephesians 4:29 tells us, "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." First Peter 3:10 declares, "For, whoever would love life
and see good days must keep his tongue from evil and his lips from
deceitful speech." James 3:9-12 summarizes the issue: "With the tongue
we praise our Lord and Father, and with it we curse men, who have been
made in God's likeness. Out of the same mouth come praise and cursing.
My brothers, this should not be. Can both fresh water and salt water
flow from the same spring? My brothers, can a fig tree bear olives, or
a grapevine bear figs? Neither can a salt spring produce fresh water."

James makes it clear that the lives of Christians—the
"brothers"—should not be characterized by evil speech. By making the
analogy of both salt water and fresh water coming from the same spring
(which is uncharacteristic of springs), he makes the point that it is
uncharacteristic for a believer to have both praise and cursing come
from his/her mouth. Nor is it characteristic for us to praise God on
one hand and curse our brothers on the other. This, too, is
uncharacteristic of a true believer.

Jesus explained that what comes out of our mouths is that which fills
our hearts. Sooner or later, the evil in the heart comes out through
the mouth in curses and swearing. But when our hearts are filled with
the goodness of God, praise for Him and love for others will pour
forth. Our speech will always indicate what is in our hearts. "The
good man brings good things out of the good stored up in his heart,
and the evil man brings evil things out of the evil stored up in his
heart. For out of the overflow of his heart his mouth speaks" (Luke
6:45).

Why is it a sin to cuss / swear / curse? Sin is a condition of the
heart, the mind, and "the inner man" (Romans 7:22), which is
manifested in our thoughts, actions and words. When we swear and
curse, we are giving evidence of the polluting sin in our hearts that
must be confessed and repented of. Thankfully, our great God is
"faithful and just and will forgive us our sins and purify us from all
unrighteousness" (1 John 1:9). When this happens, we receive a new
nature from God (2 Corinthians 5:17), our hearts are transformed, and
our speech reflects the new nature God has created within us.