The following article is from the excellent Merriam Webster online dictionary, and although I tried to post just the link, I couldn’t get it to work. So I’m posting the article in full, and state here and now that none of it is my work. It’s all Merriam Webster, very interesting and deals with the origins and development of the word ‘forty’.
“Hello, everyone. We have an announcement. We are pleased/sorry to report that there is never a u in forty.
“That’s right: the word for the number 4 is four, but ten times that is 40, which is spelled forty. This is true in all of the vast English language, despite rumors that users of British English like the word to resemble colour (they don’t), and despite the frequent appearances of the misspelling out and about.
“In related facts, the number 14 keeps the u: it’s written as fourteen. But fortieth correlates to forty, so it too goes without a u.
“There is no good explanation for why forty lacks a u that its near-relation four has. Forty simply is, as American English Spelling author D.W. Cummings calls it, an “ill-formed but accepted spelling.” It is, however, also a relatively new spelling.
“Origins and Spelling Variants
“While the word forty dates back to the language’s earliest incarnation, it had many varied spellings over the centuries, and the current spelling forty dates only to the 16th century. The Oxford English Dictionary includes a number of spellings that predate that one. From Old English (English as it existed from the 7th century to around 1100) there are the following:
“But things really got going in Middle English—English as it existed between the 12th and 15th centuries. In texts from that period the OED notes the following spellings:
feowerti (and fowerti)
feuwerti (and fuwerti)
“Modern English brought us other options:
“The winner, of course, is forty, nearly the last of the bunch. The logical Middle English relic fourty, hiding most of the way down that long list, lasted until the 18th century, when for reasons unknown it fell out of use. Sometimes that’s just how it goes in English.”
By The Editors. (Merriam Webster)
D.W. Cummings, American English Spelling (Baltimore and London: The Johns Hopkins University Press, 1988), pgs. 28, 31.
“forty.” OED Online, https://www.oed.com/view/Entry/73764. Oxford University Press. Accessed 9/24/2019.