Whether it is the cardinal sin, or the (cuddly?) slow-moving animal, there is a question mark over its pronunciation. On this morning’s edition of Saturday Live on BBC Radio 4, the well-known ‘sloth-woman’, Lucy Cooke, was interviewed. She always says the name to rhyme with ‘moth’, because, as she says, there is a moth that lives on the sloth, and this is the /slɒθ mɒθ/, not the /sləʊθ məʊθ/. Obviously this is a silly argument, as we all know that spelling in English is so apparently inconsistent that there is no reason why it should not be a /sləʊθ mɒθ/. She further said that she distinguished it from the sin, which she pronounced /sləʊθ/. A listener texted, or tweeted, that as the animal was named for the sin, this argument was also unsatisfactory. Then Sir David Attenborough got in on the act, letting the programme known that he always called both the animal and the sin /sləʊθ/. The Oxford English Dictionary was also quoted, and this has /sləʊθ/ as the usual British pronunciation, but /slɔθ, sloʊθ, slɑθ/ the American (given in that order, presumably of frequency heard).
I have always said /sləʊθ/. The etymology of ‘sloth’ is the adjective ‘slow’ plus the nominalizing suffix ‘-th’, as in ‘width, breadth, depth, length, warmth, coolth’. It seems that the vast majority of these have a short vowel which is derived from a long vowel in the root adjective. Whatever the form in Middle English, therefore (the OED’s earliest example is from ca 1175 – ‘slauðe’), there must be a strong analogical pull in ‘sloth’ towards shortening its vowel too. Added to which, more of the words ending in –oth have a short vowel: broth, cloth, Goth, moth, the Egyptian god Thoth, while both and loth are the only other words to come to mind immediately with /əʊ/. The loss of the letter ‘w’ has made us forget the word’s compound morphological origins.