"If any offend not in words he is a perfect man " (James, iii. 2). Be careful, then, never to let slip an indecent word: for although you do not speak it with an ill intention, yet it may be hurtful to those that hear it. An evil word falling into a weak heart spreads itself like a drop of oil falling on linen: nay, it sometimes seizes on the heart in such a manner as to fill it with a thousand impure thoughts and temptations to sin, for as the poison of the body enters by the mouth, so the poison of the soul enters by the ear, and the tongue that utters it is a murderer.
— St. Francis de Sales, Introduction to the Devout Life, Chapter XXVII, Modesty in our words