This is the point of contact from the coxix tho the anus of one's buttocks. As the buttocks can be spread apart, the contact forms a 'crease'. This crease in in one's buttocks is known as the 'Batty Crease'. It is a colloquialism that can be used as a noun or an adjective.
'You're a dirrrty batty crease bitch!'(in terms of adjective) 'Yo bitch's batty crease is masssive man!'(noun)
by Me's gonna beat up dem batty mc's at the top. Ba July 03, 2003
The point at which ones thighs meet at the back of the body, usually referred to as the buttocks. The "batty crease" is therefore the crack in ones buttocks
"That yattie over there, she got some big up batty crease. Me see it like the grand canyon, man."
by MC Barebreeze, MC Bareflex, MC Barefassy March 06, 2003