The story
Shortened from various (often vulgar) phrases for parts of the anatomy of a gnat, a very tiny insect. These include gnat's arse/gnat's ass, gnat's chuff, gnat's prick, and gnat's whisker.
Etymology from Wiktionary, CC BY-SA 4.0.
gnat's
Today it means: A very tiny thing or distance; a whit; a jot; something of no consequence. Full definition →