probably from Old English trymman "strengthen, make ready," from trum "strong, stable," from Proto-Germanic *trumaz; said to be cognate with Sanskrit drumah "tree," Greek drymos "copse, thicket," drys "tree, oak," and Old English treow (see tree). Examples in Middle English are wanting.
Original sense is preserved in nautical phrase in fighting trim (see trim (n.)). Meaning "make neat by cutting" is first recorded 1520s; that of "decorate, adorn" is from 1540s. Sense of "reduce" is attested from 1966. The adjective sense of "in good condition, neat, fit" is attested from c.1500, probably ultimately from Old English adjective trum.