1610s, "an image of the penis," from Latin phallus, from Greek phallos "penis," also "carving or image of an erect penis (symbolizing the generative power in nature) used in the cult of Dionysus," from PIE *bhel-no-, from root *bhel- (2) "to blow, swell" (source also of Old Norse boli "bull," Old English bulluc "little bull," and possibly Greek phalle "whale"). Used of the penis itself (often in symbolic context) from 1924, originally in jargon of psychoanalysis.
