means To cut away small amounts of something to make it neat, balanced, or the right size — or, as an adjective, to be sleek and well-proportioned.

from From Old English 'trymman' or 'trymian,' meaning to make firm, strengthen, arrange, or prepare. The word seems to have settled into the sense of putting things in good order, then narrowed to the neatening kind of order — the tidy haircut, the balanced ship (a vessel 'in trim' rides level in the water, a nautical sense still alive today). The thread running through every meaning is readiness through arrangement: making something fit, fixed, and right.