The male organ of copulation in higher vertebrates, homologous with the clitoris. In mammals, it also serves as the male organ of urinary excretion.
Or as other folk would have it...
An appendage attached to the male groin that can be very excitable or very vulnerable.
Penises should be approached with caution.
Sometimes they dribble,
Sometimes they spit at you,
Sometimes they spread nasty germs,
and sometimes they spread pregnancy.
They are often found wearing condoms.