Barking mad

Utterly insane or wildly eccentric, acting with such unhinged fervor it's like a dog barking nonstop at nothing-frantic and absurd.

From 20th-century Britain, possibly tied to rabid dogs; 'barking' at imaginary foes signaled derangement, noted in 1920s slang. Alternatively, it echoes 'barking up the wrong tree,' but with madness over error; a 1933 novel uses 'barking mad' for a raving lord. It reflects interwar Britain's knack for quirky insult, possibly amplified by asylum tales or the noisy chaos of barking kennels, growing into a zippy idiom for nutty excess.