"military officer who assists superior officers," c. 1600, from Latin adiutantem (nominative adiutans), present participle of adiutare "to give help to, help zealously, serve," frequentative of adiuvare (past participle adiutus) "help, assist, aid, support," from ad "to" (see ad-) + iuvare "to help, give strength, support," which is perhaps from the same root as iuvenis "young man" (see young (adj.)).
French adjudant, earlier ajudant (early 18c.) is from Spanish cognate ayudante. Related: Adjutancy. The adjutant bird is the name given by the English in Bengal to a large type of Indian stork, so called for its "stiff martinet air" [Century Dictionary].