The Collaborative International Dictionary of English v.0.48:

Slabber \Slab"ber\, v. i. [imp. & p. p. Slabbered; p. pr. &
   vb. n. Slabbering.] [OE. slaberen; akin to LG. & D.
   slabbern, G. schlabbern, LG. & D. slabben, G. schlabben,
   Icel. slafra. Cf. Slaver, Slobber, Slubber.]
   To let saliva or some liquid fall from the mouth carelessly,
   like a child or an idiot; to drivel; to drool. [Written also
   slaver, and slobber.]
   [1913 Webster]