What's the "een" in Halloween?
c.1300, from a Scandinavian source akin to Old Norse rotinn "decayed," past participle of verb related to rotna "to decay," from Proto-Germanic stem *rut- (see rot (v.)). Sense of "corrupt" is from late 14c.; weakened sense of "bad" first recorded 1881. Rotten apple is from a saying traced back to at least 1528: "For one rotten apple lytell and lytell putrifieth an whole heape." The Rotten Row in London and elsewhere probably is from a different word, but of uncertain origin.
Deplorable; nasty; inept and bungled: This is a rotten situation altogether (1880+)