1467, alteration of potynger "small dish for stew," from potage (see pottage) by infl. of porridge, with intrusive -n- by 1538 (cf. passenger, messenger).
c.1532, "soup of meat and vegetables," alteration of pottage, influenced by M.E. porreie, from O.Fr. poree "leek soup," from por "leek," from L. porrum "leek." Association with oatmeal is 1643, first in Scottish.
c.1300, "food for one meal, pottage," from O.Fr. mes "portion of food, course at dinner," from L.L. missus "course at dinner," lit. "placing, putting (on a table, etc.)," from mittere "to put, place," from L. mittere "to send, let go" (see mission). Sense of "mixed food" led to contemptuous use for "jumble, mixed mass" (1828), and figurative sense of "state of confusion" (1834), as well as "condition of untidiness" (1851). Meaning "communal eating place"(esp. a military one) is first attested 1530s, from earlier sense of "company of persons eating together" (early 15c.), originally a group of four. To mess with "interfere, get involved" is from 1903; mess up "make a mistake, get in trouble" is from 1933, both originally Amer.Eng. colloquial.