gnash one's teeth

Express a strong emotion, usually rage, as in When Jonah found out he was not going to be promoted, he gnashed his teeth. This expression is actually redundant, since gnash means “to strike the teeth together.” Edmund Spenser used it in The Faerie Queene (1590): “And both did gnash their teeth.” [Late 1500s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.