Douglas Harper's Etymology Dictionary
before vowels plagi-, word-forming element meaning "slanting, oblique," from comb. form of Greek plagios "oblique, slanting," from plagos "side," from PIE *plag- "flat, spread," from *plak- (1) "to be flat" (see placenta).
pre. (context chiefly science English) Forming compounds nouns and adjectives having the sense, ‘slanted, oblique’.