c. 1300, "not filled, held, or occupied," from Old French vacant "idle, unoccupied" (of an office, etc.), from Latin vacantem (nominative vacans), "empty, unoccupied," present participle of vacare "be empty" (from PIE *wak-, extended form of root *eue- "to leave, abandon, give out"). Meaning "characterized by absence of mental occupation" is from 1570s. Related: Vacantly.
c. 1600, "to expose to moonlight;" later "idle about, wander or gaze moodily" (1836), "move listlessly" (1848), probably on the notion also found in moonstruck. The meaning "to flash the buttocks" is recorded by 1968, U.S. student slang, from moon (n.) "buttocks" (1756), "probably from the idea of pale circularity" [Ayto]. See moon (n.). Related: Mooned; mooning.