A fresco at the entrance of the House of the Vettii in Pompeii that depicts the god Priapus weighing his semiturgid phallus against a bag of coins has plausibly been interpreted as an apotropaic image, protecting the domus against the baneful influence of the evil eye. This article points to humorous elements of the fresco that have been largely overlooked in previous scholarship and suggests that these elements were understood to enhance the apotropaic effectiveness of the image. Humorous elements in the fresco discussed include artistic disproportionality (i.e., the grossly enlarged phallus), the use of inversion (a semiturgid rather than fully erect phallus), partial gender reversal (Priapus is dressed in matronly garb), and the transformation of the function of Priapus’s member from penality to mensuration.