before vowels plagi-, word-forming element meaning "slanting, oblique," from Greek plagios "oblique, slanting," from plagos "side," from PIE *plag- "flat, spread," variant form of root *plak- (1) "to be flat."

word-forming element meaning "level, flat, plane," from Latin plani-, from planus "flat, level" (from PIE root *pele- (2) "flat; to spread").

alternative form of plani- "flat, level" (based on Latin planus), but an identical word-forming element is used in sciences as a combining form of Greek planos "wandering" (see planet).

also pleo-, word-forming element meaning "more," from Greek pleiōn "larger, greater in quantity, the more part, very many" (comparative of polys "much"), from PIE *ple- (source also of Latin plere "to fill," plebes, "the populace, the common people;" Greek plēthein "be full," plērēs "full"), possibly a variant of root *pele- (1) "to fill."

before vowels pleur-, word-forming element meaning "pertaining to the side; pertaining to the pleura," from Greek pleura "the side, the ribs" (see pleura).

consonant sound in some English words derived from Greek. The p- typically is silent in English but pronounced in French, German, Spanish, etc.

It is to be desired that it were sounded in English, also, at least in scientific and learned words; since the reduction of pneo- to neo-, pneu- to new-, and pnyx to nix, is a loss to etymology and intelligibility, and a weakening of the resources of the language. [OED]