tailspin

Short definition from Wiktionary

Noun

1. 1
   The rapid, uncontrollable descent of an aircraft in a steep spiral.
2. 2
   A severe mental or emotional collapse; emotional breakdown.
3. 3
   Any sharp, sustained, often uncontrollable descent or decline.

Verb

1. 1
   Of an aircraft: to go into a rapid, uncontrollable descent in a steep spiral.
2. 2
   To go into a sharp, sustained, often uncontrollable descent or decline.