wen an erection occurs whilst a penis is inside the body/tummy, usually caused by a baby who was a fetus in fetu that grows up with the "fetu" (or baby inside the baby) still inside their body. if the fetu is a boy it can still get an erection if the grown body is arroused causing a "penummy" (or penis tummy)
linda:"oh my god jenny! what's that sticking out of your tummy?"
jenny: "oh that's just my penummy."