Slothful comes from sloth (from Old English slǣwth 'laziness', from slāw 'slow') plus the suffix -ful. The historical chain includes Old English slāw. The quoted older senses, "laziness" and "slow", supply the semantic anchors for slothful. The formation is best read as suffixal and borrowed: the word slothful carries its older form-history into a compact English lexical shape. In present English it is used as a adj, "reluctant to work or exert oneself; lazy and indolent". English derivatives include slothfully and slothfulness; related English forms include sloth and slow. The result is a adj whose etymology preserves both the older form and the later English specialization.