1570s, "a wandering, a roaming journey," from Italian vagare or directly from Latin vagari "to wander, stroll about, roam, be unsettled, spread abroad," from vagus "roving, wandering" (see vague). The infinitive appears to have been adopted in English as a noun and conformed to nouns in -ary, "but this can hardly be explained except as an orig. university use" [Century Dictionary]. Current meaning of "eccentric notion or conduct" (1620s) is from notion of mental wandering. Related: Vagaries.