Origin: 1400–50; late Middle English parboylen to boil partly, (rarely) to boil fully < Middle French parboillir < Late Latin perbullīre to boil through and through (see per-, boil); change of meaning by confusion of par- with part
1381, from O.Fr. parboillir, from M.L. perbullire "to boil thoroughly," from L. per "through, thoroughly" + bullire "to boil" (see boil (v.)). Main modern meaning "boil partially" (c.1440) is by mistaken association of the prefix with part.