swear like a sailor
To use profanities or vulgar language very freely and fluently. (An allusion to the rough language presumed to be used by military personnel.) My little sister has been swearing like a sailor ever since she started learning bad words. My granny is the sweetest old lady you'll ever meet, but she swears like a sailor when she gets on the topic of something or someone she doesn't like.
swear on (one's) mother's grave
To make a very serious, solemn pledge, especially that one is telling the truth. Janet has sworn on her mother's grave that she wasn't the one to betray me, and I believe her. I swear on my mother's grave that if I have a chance to help your campaign, I will.
sworn enemy
A person, group, or organization for whom one holds a particularly intense, deep-seated hatred. Our environmentalist lobby group is heading out to protest against Big Oil, our sworn enemy.