A term for the vagina, in the context of oral sex with one of the highest qualities of purity, cleanliness and superb taste. The type of vagina that one would perform oral sex with the same way one would enjoy the world's finest prime rib, which is the reason for steak being included. Twank = vagina. Steak = grade A prime rib.
I can hardly wait arriving at my girlfriend's place after work. She promised me a triple serving of Twank-steak this evening!