stitch is a shorter term in place of the longer phrase of "stupid ass bitch" or "Stupid Bitch". Hence STupid (Ass can be insterted anywhere if need be) then bITCH, ending up with STITCH.
Lindsay: Hey Matt!
Matt: Yes Lindsay?
Lindsay: Watch out for those kids, they are definately STITCHES and should not be trusted.
Matt: Instead of snitches get STITCHES its STITCHES get creepers. Lets get them!