parboil v.

"through, by means of," 1580s (earlier in various Latin and French phrases, in the latter often par), from Latin per "through, during, by means of, on account of, as in," from PIE root *per- (1) "forward," hence "through, in front of, before, first, chief, toward, near, around, against."

early 13c. (intransitive) "to bubble up, be in a state of ebullition," especially from heat, from Old French bolir "boil, bubble up, ferment, gush" (12c., Modern French bouillir), from Latin bullire "to bubble, seethe," from PIE *beu- "to swell" (see bull (n.2)). The native word is seethe. The figurative sense, of passions, feelings, etc., "be in an agitated state" is from 1640s.

I am impatient, and my blood boyls high. [Thomas Otway, "Alcibiades," 1675]

The transitive sense "put into a boiling condition, cause to boil" is from early 14c. The noun is from mid-15c. as "an act of boiling," 1813 as "state of boiling." Related: Boiled; boiling. Boiling point "temperature at which a liquid is converted into vapor" is recorded from 1773.

"by, for," mid-13c., from Old French par, per, from Latin per (see per). It figures in some French phrases borrowed into English and in the formation of some words (parboil, pardon, parvenu). In some older borrowings from French it has been re-Latinized to per- (perceive, perfect, perform, pertain).