means The act of cutting away small or excess parts to neaten, shorten, or shape something — or, as a noun, the bits cut off, or the decorative additions that finish a thing off.

from From the Old English verb 'trymman' (also 'trymian'), meaning to strengthen, make firm, prepare, or set in order — the same root that gave us 'trim' as a word for tidy and well-arranged. The leap from 'making firm and ready' to 'cutting back to neatness' came naturally: to ready a ship, a hedge, or a sail was to put it in proper order, and proper order often meant snipping off the surplus. By the time it picked up the '-ing' to name the action (and the leftovers), trimming carried both senses at once — adding finish and removing excess, two ways of arriving at the same tidy end.