I recall having an argument with my first wife many years ago about this word. She insisted that it meant to boil partly, I that it meant to boil thoroughly. Well, she was right and I was wrong. OED marks the ‘boil thoroughly’ sense as obsolete (although I could have sworn I’ve heard or read modern examples of it). I ceded the field then but the question is complex. For instance she used the word in the sense ‘to bring almost to the boil’ whereas the OED definition is ‘to cook partially by boiling’, which is a horse of a very different colour. Interestingly the popular misidentification of the prefix par- as coming from part rather than Latin per- goes way back to Anglo-Norman and led to the sense part-boil rather than through-boil. Here’s OED (I’ve made sure to include the 1670 cite referenced in the etymology):
Etymology: < Anglo-Norman parboillir, perboillir to cook partially by boiling, to cook thoroughly by boiling, and Old French, Middle French parboulir to cook thoroughly by boiling (French parbouillir , now regional in sense ‘to boil, to boil down’) < post-classical Latin perbullire to boil thoroughly (6th cent.) < classical Latin per- per- prefix + bullīre boil v. In Anglo-Norman the prefix was apparently identified with par part n.1 (compare part adv., and for later evidence of the same identification compare quot. 1670 at sense 1).
1. trans. To cook partially by boiling.
1381 Diuersa Servicia in C. B. Hieatt & S. Butler Curye on Inglysch (1985) 62 Pecokys and pertrigchis schul ben yperboyld & lardyd..&etyn wyþ gyngeuyr.
a1450 in T. Austin Two 15th-cent. Cookery-bks. (1888) 6 Take fayre caboges..an parboyle hem in fayre water.
?a1475 Noble Bk. Cookry 31 To mak yonge pessene, tak pessen and par boille hem in water.
1530 J. Palsgrave Lesclarcissement 652/1 It muste be parboyled first and than baken: il le fault parbouyllyr premier et puis le mettre cuyr au four.
1670 T. Blount Glossographia (ed. 3) , Par-boile, Part-boil, to boil in part not fully.
2. trans. fig. and in figurative contexts (freq. in hyperbolical use, with reference to overheating).
1565 T. Stapleton tr. Bede Hist. Church Eng. iv. ix. f. 122v, It might al be perboyled out by the fire of long tribulation.
1566 T. Drant tr. Horace Medicinable Morall sig. Eiijv, My harte in choller perboylde was.
1616 B. Jonson Every Man in his Humor (rev. ed.) iv. v, in Wks. I. 45 They should haue beene perboyl’d, and bak’d too, euery mothers sonne.
3. trans. To boil thoroughly. Obs.
1611 R. Cotgrave Dict. French & Eng. Tongues, Pourbouiller, to parboile throughly.
a1655 T. T. de Mayerne Archimagirus Anglo-Gallicus (1658) v. 2 Take the Hare and par-boyl him, then take all the flesh from the bone.
Fascinating that the figurative sense seems to hark back to the ‘boil thoroughly’ meaning. Now if I could only hunt up my ex and nitpick this with her! I’m sure she would be most gratified.