Is it farther or further?
c.1400, probably of imitative origin; cf. Frisian slobberje "to slurp," Middle Low German slubberen "slurp," Middle Dutch overslubberen "wade through a ditch." Related: Slobbered; slobbering. As noun from c.1400 as "mud, slime," 1755 as "saliva." Congreve has slabber (v.), from Middle Dutch slabberen.