1. Literally, not in the correct key; out of tune. Hyphenated if used before a noun. I think your voice is a bit off key. Overall, the performance was very enjoyable, but there was one off-key brass player that was driving me nuts the whole night.

2. By extension, at odds with what is normal, standard, or expected. Hyphenated if used before a noun. There was something off key about the way he spoke that made me think that something was very wrong. Her character is an off-key caricature of modern superheroes.