Well there goes that entry. I was about to start up about how then, if the underlying meaning is fellatious, how can this have become such an acceptable saying that entire dish network commercials be centered around it. Then I make the mistake of trying to validate my rantings and find this from The Online Etymology Dictionary:
O.E. sucan, from PIE root *sug-/*suk- of imitative origin (cf. O.S., O.H.G. sugan, O.N. suga, M.Du. sughen, Du. zuigen, Ger. saugen "to suck;" L. sugere "to suck," succus "juice, sap;" O.Ir. sugim, Welsh sugno "to suck"). Meaning "do fellatio" is first recorded 1928. Slang sense of "be contemptible" first attested 1971 (the underlying notion is of fellatio). Suck eggs is from 1906. Suck hind tit "be inferior" is Amer.Eng. slang first recorded 1940.
So you really do learn something every day. Even days when by 3:15 you've only been up for four hours and haven't bathed.