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)