go spare
idiomatic phrasal verb

- to become very angry

Cambridge Dictionary

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ORIGIN

The verb "spare" goes back to the late 14th century and referred to "kept in reserve, not used, held for extra need." It's closely related to the German "sparen," to save, as in money or resources. In the 15th century it took on the additional sense of "lacking in substance, lean, thin, poor."

The main sense of something in reserve is still widely used, such as in spare part, spare change (money) or spare tyre. The phrase "going spare" is also sometimes used in the sense of extra or available for re-use: In a statement Uber said: "There are a whole three seats going spare if it’s just you and your driver travelling."

In an interesting evolution, "go spare" developed the additional meaning of "very angry" in the 1940s. It's believed this primarily British idiom is connected to the use of spare to mean "idle, unemployed." Thus someone who is suddenly out of work would not only be idle, but also angry about it. This usage then expanded to any situation in which someone becomes irate or enraged.

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SYNONYMS

angry, enraged, furious, galled, incensed, irate, mad;
go ballistic, go berserk, go crazy, go haywire, go off the deep end, go nuts, go wild, blow a gasket, freak out, lose it, hit the ceiling

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Practice OWAD in a conversation:

"If we are late for the meeting again, the boss will GO SPARE!"