c. 1300, "make straight; direct, guide, control; prepare for cooking," from Old French dresser, drecier "raise (oneself); address, prepare; lift, raise, hoist; set up, arrange, set (a table), serve (food); straighten, put right, direct," from Vulgar Latin *directiare "make straight," from Latin directus "direct, straight," past participle of dirigere "set straight," from dis- "apart" (see dis-) + regere "to direct, to guide, keep straight" (from PIE root *reg- "move in a straight line").
Sense of "decorate, adorn" is from late 14c., as is that of "put on clothing." The older sense survives in military dress ranks "align columns of troops." Of males, in reference to the position of the sex organ in trousers, by 1961.
Dress up "attire elaborately, put on one's best clothing" is from 1670s; dress down "wear clothes less formal than expected" is by 1960. Transitive use of dress (someone) down, "scold, reprimand," is by 1876, earlier simply dress (1769), in which the sense is ironical. In Middle English, dress up meant "get up" and dress down meant "to kneel." Related: Dressed; dressing.