by 1969, originally British underworld and juvenile delinquent slang, short for aggravation in a colloquial sense of "trouble or disturbance provoked by aggressive behavior or harassment" (by 1939).
Entries linking to aggro
aggravation n.
late 15c., "an increasing in gravity or seriousness," from French aggravation, from Late Latin aggravationem (nominative aggravatio), noun of action from past-participle stem of Latin aggravare "make heavier," figuratively "to embarrass further, increase in oppressiveness," from ad "to" (see ad-) + gravare "weigh down," from gravis "heavy" (from PIE root *gwere- (1) "heavy"). The sense of "irritation" is from 1610s.