swear like a trooper
Freely utter profanity or obscenity, as in The teacher was shocked when she heard one of the fathers begin to swear like a trooper. The troopers in this term were the cavalry, who were singled out for their swearing from the early 1700s on.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.