"pastry," mid-14c. (probably older; piehus "bakery" is attested from late 12c.), from Medieval Latin pie "meat or fish enclosed in pastry" (c. 1300), perhaps related to Medieval Latin pia "pie, pastry," also possibly connected with pica "magpie" (see pie (n.2)) on notion of the bird's habit of collecting miscellaneous objects. Figurative of "something to be shared out" by 1967.
According to OED, not known outside English, except Gaelic pighe, which is from English. In the Middle Ages, a pie had many ingredients, a pastry but one. Fruit pies began to appear c. 1600. Figurative sense of "something easy" is from 1889. Pie-eyed "drunk" is from 1904. Phrase pie in the sky is 1911, from Joe Hill's Wobbly parody of hymns. Pieman is not attested earlier than the nursery rhyme "Simple Simon" (c. 1820). Pie chart is from 1922.