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).

"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.