Q: Is the slow-moving sloth that lives in trees the source of our word for laziness? Or vice versa?
A: Vice versa. The noun “sloth” (idleness, indolence, or laziness) is derived from the Old English adjective sláw (slow), according to the Oxford English Dictionary.
The earliest OED example for “sloth” (spelled slauðe in Middle English) is from the Lambeth Homilies (circa 1175): “Þe licome luuað muchele slauðe and muchele etinge” (“The body loves great sloth and great eating”).
The dictionary’s first example for “sloth” used in reference to the “arboreal mammal of a sluggish nature” is from Purchas His Pilgrimage, a 1613 book by the Anglican cleric Samuel Purchas about his travels and observations:
“The Spaniards call it … the light dog. The Portugals Sloth.”
(Purchas is mistaken here. He apparently confused perro, Spanish for “dog,” with perezoso, which means both “lazy” and the arboreal “sloth.” In Portuguese, preguiça means both “laziness” and the “sloth” that lives in trees.)
The OED citation is from a note to an interesting description of the sloth’s habits, in a section of the Purchas book about the wildlife of Brazil:
“There is a deformed beast of such slow pace, that in fifteene dayes it will scarse goe a stones cast. It liueth on the leaues of trees, on which it is two dayes in climing, and as many in descending, neither shouts nor blowes forcing her to amend her pace.”
Getting back to human sloth, writers don’t mention it much these days, though Mike Dover cites “sloth” in his 2016 book, Dante’s Infinite Monkeys, as one of the seven deadly sins of our digital lives:
“The Internet, and technology in general, have provided new ways for wrath, lust, gluttony, sloth, pride, envy and greed to insert themselves into our lives.”