sloth

Sloth comes from Middle English slouthe or slewthe, meaning laziness, which came from Old English slǣwþ, meaning sloth, indolence, or torpor. That in turn traces back to Proto-Germanic *slaiwiþō, meaning slowness or lateness, built from the root of slow plus the suffix -th, which turns an adjective into an abstract noun.

At its core, sloth has always meant the same thing: a state of slowness or inertness, first captured directly in Old English and never really drifting from that sense.

Sloth is built the same way as slow plus -th, so the two words share a root — sloth is essentially 'slowness' turned into an abstract noun. It's also cognate with Scots sleuth, which likewise meant sloth or slowness.