means A wire stretched low across a path so that anyone who catches it triggers a trap, alarm, or explosive — and by extension, any small thing that sets off a sudden chain of consequences.

from A transparent compound of 'trip' and 'wire.' 'Trip' comes from Middle English 'trippen,' meaning to step lightly or stumble — itself likely borrowed from Old French 'tripper,' to skip or dance — and 'wire' descends from Old English 'wir,' a drawn metal thread, a cousin of words across the Germanic languages. The two were welded together as a practical military and trapping term, naming the literal thing: a wire you trip on. The figurative sense — a hidden trigger that sets events in motion — followed naturally, because nothing teaches metaphor faster than learning the hard way.