early 14c., "to be agreeable," from Old French plaisir "to please, give pleasure to, satisfy" (11c., Modern French plaire, the form of which is perhaps due to analogy of faire), from Latin placere "to be acceptable, be liked, be approved," related to placare "to soothe, quiet" (source of Spanish placer, Italian piacere), possibly from PIE *plak-e- "to be calm," via notion of still water, etc., from root *plak- (1) "to be flat" (see placenta).
Meaning "to delight" in English is from late 14c. Inverted use for "to be pleased" is from c.1500, first in Scottish, and paralleling the evolution of synonymous like (v.). Intransitive sense (e.g. do as you please) first recorded c.1500; imperative use (e.g. please do this), first recorded 1620s, was probably a shortening of if it please (you) (late 14c.). Related: Pleased; pleasing; pleasingly.
Verbs for "please" supply the stereotype polite word (e.g. "Please come in," short for may it please you to ...) in many languages (French, Italian), "But more widespread is the use of the first singular of a verb for 'ask, request' " [Buck, who cites German bitte, Polish proszę, etc.]. Spanish favor is short for hace el favor "do the favor." Danish has in this sense vær saa god, literally "be so good."