From Middle English sloberen, borrowed from Middle Dutch slobberen (> Modern Dutch slobberen (“to slobber”)), related to West Frisian slobberje (“to slurp”), German Low German slubbern (“to slobber”). Doublet of slabber and slaver. Compare also German schlabbern (“to slobber”). At least in the sense of "saliva", apparently not related to English slob, from Irish slaba (“mud”).