The Collaborative International Dictionary of English v.0.48:
off-key \off-key\ adj. & adv. Deviating from the proper pitch; -- of a musical note or series of notes; as, an off-key note; to sing off-key. Syn: false, sour. [WordNet 1.5]WordNet (r) 3.0 (2006):
off-key adj 1: inaccurate in pitch; "a false (or sour) note"; "her singing was off key" [syn: false, off-key, sour]