auxiliary (adj.)

"assisting, giving support," hence "subsidiary, additional," c. 1600, from Latin auxiliaris "helpful, aiding," from auxilium "aid, help, support," related to auctus, past participle of augere "to increase," from PIE root *aug- (1) "to increase."

c. 1600, "foreign troops in service of a nation at war," from auxiliary (adj.). The Latin adjective also was used as a noun in this sense. In grammar, "a verb used in forming phrases with other verbs and indicating mode or tense," 1762, from the adjective in this sense (1670s). Related: Auxiliaries.