Short for 'customer'- drug dealer slang used to denote either a customer in general or a customet who would pay higher then average prices for the drugs. Can also be used a s a verb 'to custie someone'.
'Super' is often added to strengthen the word (ie a supercustie)
Mike is such a custie...he paid $60 for this eighth of weed.
As verb: I totally custied Mike. He got custie.