From Longman Dictionary of Contemporary Englishquitterquit‧ter /ˈkwɪtə $ -ər/ noun [countable] informalSTOP DOING something someone who does not have the determination or courage to finish something that is difficult
Examples from the Corpus
quitter• In nearly 3,000 years with this company, I have never been known as a quitter.• I'm not a quitter, but this job is starting to affect my health.• But President Tucker was not a quitter, and he got right back on the horse.• But these were early days, and Shelley wasn't a quitter.• I was a coward, a damned quitter.• Even Clinton's enemies have to admit that, with the bruising he has taken, he is no quitter. 