'Poon' is an abbreviated form of 'poontang', which is a Ilokano word which translates as 'whore-
fuck'. This is in contrast to just a regular
fuck, where whore-
fuck means that there is some guaranteed action that will happen, so that prospective client will not just be throwing away their cash. This term often refers to the smell from 10 other guys dicks having been in it already.