In Greek mythology, Priapus was a fertility god, protector of livestock, fruit plants, gardens and male genitalia. (Roman equivalent: Mutinus Mutunus) He was a son of Dionysus and Aphrodite. Sculptures of Priapus with large, ithyphallic genitalia were placed on gardens and fields to guarantee an abundant crop. He was much more popular in Roman mythology than in Greek.
He tried to rape Lotis, and she was changed into a lotus plant to protect her. In Ovid's Fasti, an imbibed Lotis is attempted by the aroused Priapus, at which time one of Silenus's donkeys (he was hosting the feast) with "raucous braying" revealed Priapus' intentions and the entire party had a good laugh at his expense. To repay the donkey for the embarrassment, the annual feast (sometime during May) of Priapus is begun by the sacrifice of a donkey to the diminutive and ironic garden-god.
The medical condition priapism gets its name from Priapus.