flunk(noun, verb) noun1. failure to reach a minimum required performance- he got two flunks on his reportSimilar word(s): failingDefinition categories: act, failure verb1. fail to get a passing gradeSimilar word(s): bomb, failDefinition categories: socialSentences with flunk as a verb:- He flunked math, again.- Unsatisfied with Fred's progress, the teacher flunked him.