Part of speech: Noun.
The ultimate insult. A person labeled as a "Bitchmilk" is one who is whiny, snobbish, complaining, pessimistic, cowardly, bitchy, really bitchy, and cunt-ish.
John was such a bitchmilk last night for not taking that last shot!
In most basic terms, the opposite of a badass. It describes one who, while drinking with friends, does not have the testicular fortitude to finish one's drink.
He couldn't finish his fifth Jagerbomb. What a bitchmilk.