"Adjunct" comes straight from Latin, and that's basically why it looks and sounds the way it does today. The Latin word "adiunctus" meant "closely connected, joined, united," and it came from the verb "adiungere" — "to join to" — a relative of our word "adjoin." English borrowers in the 1580s took this Latin past participle almost unchanged, which is why "adjunct" keeps that Latin "-junct" spelling rather than being reshaped into something more native-looking.
The word entered English doing double duty. As a noun, starting in the 1580s, it described something added onto a main thing without being essential to it — an extra, a tack-on. Just a decade later, in the 1590s, it also became an adjective meaning "joined with another in office or action," often carrying a whiff of subordinate status, though that hint of lesser rank wasn't really built into the original Latin meaning — it crept in through usage. The familiar modern phrase "adjunct professor," describing a teacher attached to a department without full permanent status, shows up in American English by 1826, a natural extension of that older sense of being connected to something central while remaining somewhat apart from it.
adjunct (n.) 1580s, "something added to but not an essential part of (something else)," from Latin adiunctus "closely connected, joined, united" (as a noun, "a characteristic, essential attribute"), past participle of adiungere "join to" (see adjoin ). also from 1580s adjunct (adj.) "united with another in office or action," 1590s, from Latin adiunctus "closely connected, joined, united," past participle of adiungere "to join to," usually with a notion of subordination, but this is not etymological (see adjoin ). Adjunct professor is attested by 1826, American English. also from 1590s