nosedive

noun Also nose dive.

1. a plunge of an aircraft with the forward part pointing downward.
2. a sudden sharp drop or rapid decline: a time when market values were in a nosedive.

verb (used without object), nose·dived or nose·dove, nose·dived, nose·div·ing. Also nose-dive.

3. to go into a nosedive: a warning that prices might nosedive.