a man or woman who uses their teeth when giving a blow job. Hence the name "beaver."
Ryan: "man my fucking dick hurts. She gives really bad head."
Richard: "what do you mean?"
Ryan: "she was a fucking' beaver."
a girl, or (homosexual) man who wants to give a man head (aka blow job) very badly, then is a beaver (uses teeth when giving head; very bad).
John: "hey did she end up giving you head last night?"
John: "was she good?"
Steve: "hell no. She was an Eager McBeaver."