A person's
crotch is, as
Webster 1913 suggests, the region wherein his or her
legs meet. On the
human body, the most visible features of the crotch are the
genitalia, or external
reproductive organs: the
penis and
scrotum in men, and the
vulva in women. The crotch might be regarded as extending as far back as the
anus, though it certainly stops short of the
buttocks.
The word "crotch" is rarely used to refer pleasantly to the region in question. Perhaps because of its earthy, Anglo-Saxon sound, people tend to use it in referring to things they find painful (e.g. kick in the crotch, jamming a pair of scissors into your crotch repeatedly, crotch grab) or shameful (e.g. crotch shot, crotchless panties).
A somewhat less-charged synonym is "groin", which has the added class of being an architectural term.