Mechanism set to release at the slightest pressure

WordNet Word definitions in WordNet
n. a gun trigger that responds with little pressure

Longman Dictionary of Contemporary English Word definitions in Longman Dictionary of Contemporary English
noun EXAMPLES FROM CORPUS ▪ Getting rid of weapons reduces the risk of accidental, unauthorised, hair trigger or pre-emptive use.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Hair \Hair\ (h[^a]r), n. [OE. her, heer, h[ae]r, AS. h[=ae]r; akin to OFries. h[=e]r, D. & G. haar, OHG. & Icel. h[=a]r, Dan. haar, Sw. h[*a]r; cf. Lith. kasa.] The collection or mass of filaments growing from the skin of an animal, and forming a covering ...