"to a hair", 1733 (interdating)

[W Brewer]

<hair trigger> "a trigger that allows the firing mechanism of a firearm to
be operated by very slight pressure".
<hair> here meaning 'very delicate, sensitive' (as if  the weight of a hair
would set it off) (A nervous & jumpy <hare> springs to mind as a random
association. Elmer Fudd had a hare-trigger temper when it came to cwazy
wabbits.)

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