pomme de boeuf
Translated from french, means "apple of beef". The meaty lobe of tissue that joins the lower butt-cheek to the upper, inner/back portion of the thigh. It's not quite the butt-cheek, and not quite the thigh, but a small, meaty thing all its own.
Frank had always preferred to be bitten on the pomme de boeuf, so proper hygiene and grooming was an imperative for him.