Flunker is a slang term often used to describe someone who has failed, usually in an academic or testing setting. There are a number of different synonyms that can be used to describe someone who has flunked. Some of the most common synonyms for flunker include flop, failure, non-starter, loser, underachiever, and washout. Other similar terms that could be used to describe someone who has flunked out of a class or failed to pass an exam might include dud, reject, letdown, and disappointment. No matter which synonym you use, the underlying sentiment is one of disappointment and frustration with the individual's inability to succeed.