When an airplane falls while spinning on its tail axis, or more commonly, a state of chaos or rapid decline in any situation.
From 'tail' (rear of an airplane) plus 'spin' (rotating motion); originated as a technical aviation term and became a metaphor for loss of control.
Early pilots who got into a tailspin often died because they didn't know that easing back on the controls made it worse—you had to push forward to break the spin, completely counterintuitive to panic!
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