It is a formation of Whore, Bitch and Slut. So the person described as a "whobislut" is the biggest Whore/Bitch/Slut.
Girl 1 : OMG! You're so pretty.
Girl 2 : Awww! Thank you!
(Girl 1 turns to her friend)
Girl 1 (whispers) : She is so ugly, I so slept with her boyfriend but he wasn't that good in bed, they were so made for each other. Now, if you can excuse me I'll go sell my body on the street.
Girl 2 (overheard) : God, she is such a whobislut.