You kiss your mother with that mouth??!

If given a proper dose of intelligent thought, and willingness to be honest about our faith, we can observe that profanity (cussing) is about the most common way we defame our witness as Christians.
It is my duty, afterall, to be a living testimony of life in Christ. Should my lips then confess the risen Son and at the same time darken the ears of listeners with the tongue of demons? No.

Sounding a bit extreme?
How about this?

“Let nothing unwholesome come out of your mouths, but only what is useful for building up others, so that it might help those who listen.” Ephesians 4:29

We receive the life-giving flesh of God with our mouths! How can we then curse from that same instrument, made in the image and likeness of our creator's, who SPOKE the universe into existence?
His own mouth breathed life into Adam, and yet his children use such a mouth to curse the most common annoyances in our daily lives.

From the prayers before communion:
“I know, O Lord, that I partake of Your immaculate Body and precious Blood unworthily, and that I eat and drink judgment to myself by not discerning the Body and Blood of You ~ my Christ and God. But taking courage from Your compassion I approach You, for You have said: “He who eats My Flesh and drinks My Blood abides in Me and I in him.” Therefore have compassion, O Lord, and do not make an example of me, a sinner, but deal with me according to Your mercy…”

I admit, I let it slip now and then. And when this happens, I feel guilt and the grace of the Spirit, which I receive in the Mysteries of the Eucharist, leaves me. The Lord gave us this feeling to make us aware of ourselves. The World will tell you that cussing is not a big deal. That curse words are just an easy for us to express ourselves. Never forget, this world is upside-down. Inverted. And we were made in the image and likeness of God. God who loves mankind enough to come into the World, suffer and die to save us. His flesh and blood is our saving hope and if we believe this we should act accordingly, lest it be like a coal that burns the unworthy.