slǣwþ

Old English slǣwþ meant 'laziness' or 'indolence,' from slǣw ('slow, sluggish'). The word was moral before it was zoological. Sloth was one of the Seven Deadly Sins — acedia in Latin, the failure to do what God requires. Medieval theologians ranked it alongside pride, greed, and lust as a corruption of the soul.

When Spanish explorers encountered the slow-moving arboreal mammals of Central and South America in the 1500s, they used the term perezoso ('lazy one'). English travelers followed suit, applying their own word for laziness to the animal by the 1610s. The metaphor was irresistible: an animal that sleeps 15 hours a day, moves at 0.15 miles per hour, and grows algae on its fur.

The naming was unfair. Sloths are not lazy — they are energy-efficient. Their low metabolism is an adaptation to a diet of leaves, which provide very few calories. Moving slowly reduces energy expenditure and makes them harder for predators to detect. The green algae growing on their fur provides camouflage. Everything about the sloth is an optimization, not a deficiency.

Six living species of sloth exist, divided into two-toed and three-toed groups. Their closest living relatives are anteaters and armadillos. Giant ground sloths (Megatherium) once stood 20 feet tall and weighed four tons — they were among the largest land mammals in South America until humans arrived roughly 11,000 years ago. The deadly sin outlived the giant.