*sleubh-

Proto-Indo-European root meaning "to slide, slip." 

It forms all or part of: cowslip; lubric; lubricant; lubricate; lubricity; lubricous; sleeve; slip (n.3) "potter's clay;" sloop; slop (n.1) "semiliquid refuse;" slop (n.2) "loose outer garment;" sloven.

It is the hypothetical source of/evidence for its existence is provided by: Latin lubricus "slippery, slimy, smooth," lubricare "make slippery or smooth;" Middle Dutch slupen "to glide;" Gothic sliupan "to creep, slide;" Old English slyppe "dung."