early 14c., porcioun, "allotted part, part assigned or attributed, share," also "lot, fate, destiny," from Old French porcion "part, portion" (12c., Modern French portion) and directly from Latin portionem (nominative portio) "share, part," accusative of the noun in the phrase pro portione "according to the relation (of parts to each other)," ablative of *partio "division," related to pars "a part, piece, a share, a division" (from PIE root *pere- (2) "to grant, allot").
Meaning "a part of a whole" is from mid-14c. From late 14c. in the general sense of "section into which something is divided."
"to divide in portions," mid-14c., porciounen, from Old French porcioner "share out, divide in portions," from porcion "part, portion" (see portion (n.)), and directly from Medieval Latin portionare. Related: Portioned; portioning.