Q: Why do we “spay” a female cat or dog, but “neuter” a male? Why don’t we have a single, unisex word for the procedure?
A: You’re right that we usually say a female cat or dog is spayed, while a male is neutered.
However, “neuter” (as well as “sterilize” and “desex”) can be used with male or female pets, as can euphemisms like “fix,” “alter,” and “doctor.” (“Neuter” itself is a euphemism for “castrate” when used for males.)
Interestingly there’s no good etymological reason for restricting “spay” to females, except that’s how the term has been used since it showed up in English six centuries ago.
“Spay” is derived from the Anglo-Norman espeier (to cut with a sword), but it ultimately comes from the classical Latin spatha (a broad, flat weapon or tool) and the Greek spathe (a broad blade), according to the Chambers Dictionary of Etymology.
When the verb “spay” arrived in English in the early 15th century, it meant to remove the ovaries and destroy the reproductive power of female animals.
The earliest example in the Oxford English Dictionary is from The Master of Game, a book on hunting written around 1410 by Edward, Duke of York:
“And bycause þei [they] shuld not lese [lose] her tyme, men make hem [them] yspayed, saue þose men will kepe open to bere whelpes.”
A century later, English adopted “castrate” from castrāre, a classical Latin verb meaning to castrate, prune, expurgate, or deprive of vigor.
The first citation in the OED uses the term to mean deprive of vigor: “Ye castrate the desyres of the fleshe” (from Thomas Martin’s Traictise Marriage of Priestes, a 1554 tract challenging the marriage of Anglican priests).
In the early 17th century, “castrate” came to mean emasculate in the literal sense—that is, to remove the testicles of a man or male animal.
The first OED citation for the new sense is a 1633 religious tract by the Anglican Bishop Thomas Morton: “Origen—having read that scripture, ‘There be some that castrate themselves for the kingdom of God’… he did castrate himself.”
(The reference is to the passage in Matthew 19:12 about men who “have made themselves eunuchs for the kingdom of heaven’s sake.”)
In the early 20th century, the verb “neuter” showed up, meaning to castrate or spay an animal. It was derived from the adjective “neuter,” originally a grammatical term for words neither masculine nor feminine.
The first example for the verb in the OED is from The Book of the Cat (1903), by Frances Simpson: “A cat should be kept on low, plain diet … before being neutered.”
(In the late 19th century, the adjective “neuter” came to describe a castrated or spayed animal, as in this example from Domestic or Fancy Cats (1893), by John Jennings: “Among the principal reasons that commend neuter cats as pets, the element of non-production is chiefly important.”)
Finally, here are the other verbs mentioned above and the earliest OED dates for their use in reference to castrating or spaying animals: “alter” (1821), “desex” (1928), “doctor” (1902), “fix” (1930), and “sterilize” (1828, a human citation).
from Grammarphobia https://www.grammarphobia.com/blog/2017/07/spay-neuter.html