Fortieth

wiktionary

ref

From Middle English fourtithe, fourtiþe; equivalent to forty +‎ -eth.

etymonline

ref

fortieth (adj., n.)

"next in order after the thirty-ninth; an ordinal numeral; being one of forty equal parts into which a whole is regarded as divided;" c. 1200, fowertiþe, from Old English feowertigoða, from feowertig (see forty) + -th (1). Compare Old Norse fertugonde, Swedish fyrationde, Danish fyrretyvende.