Forty Number Names
Forty lost its u inside an Old English compound, and the shorter spelling became standard centuries before anyone thought to ask why.
Old English wrote the number fēowertig: fēower, the ancestor of four, joined to -tig, a suffix meaning a group of ten. Compounds were pronounced more lightly than standalone words, and the first syllable of fēowertig shortened early. That single sound change is the seed of the missing u.
Middle English had no fixed spelling, so scribes wrote what they heard. Manuscripts show fourty, fourtie, fortie and forty side by side, sometimes in the same text. The u-less forms matched the shortened vowel, and they grew commoner as the centuries passed.
Printing narrowed the options. After presses reached England in the 1470s, compositors had to choose one form per word, and the shorter spelling steadily gained. By the seventeenth century forty was the usual printed form, and in 1755 Johnson's dictionary entered the word under forty, without the u.
The rest of the family kept the u because the rest of the family kept the long vowel. Four, fourteen, fourth and fourfold all preserve it. Only the decade — the form where the old compound was tightest and the syllable shortest — let the letter go.
Today fourty is simply a misspelling, though a persistent one: the analogy with four is strong, and spell-checkers flag the error constantly. The reliable rule is short. Every -ty decade spells its root in full, except forty.
Further reading