early 15c., aggregacioun, originally in medicine, "formation of a pustule," from Medieval Latin aggregationem (nominative aggregatio), noun of action from past-participle stem of Latin aggregare "collect, bring together," from ad "to" (see ad-) + gregare "to collect into a flock, gather," from grex (genitive gregis) "a flock" (from PIE root *ger- "to gather"). From 1540s as "a combined whole;" 1560s as "act of collecting in an unorganized mass."
