the.com/mood swing

means A shift from one emotional state to another — often sudden and noticeable — without a clear proportional cause.

from A straightforward compound: 'mood' traces back to Old English 'mod,' meaning mind, heart, or spirit — a word that once carried real weight, covering courage and temper alike. 'Swing' comes from Old English 'swingan,' to beat or swing with force, later softening into the sense of a pendulum-like movement back and forth. Clamped together in the 20th century, likely boosted by psychiatric and medical usage, the phrase borrows the physical image of a swing's arc to describe the mind lurching from one extreme to another.