twentieth

From earlier twenteth, twentith, from Middle English twentithe, twentiþe, from Old English twēntigoþa. Equivalent to twenty + -eth. Etymological notes twentith, twenteth is the expected development from Middle English twentithe, but in time, the opaque twent- was replaced by a more transparent twenty-, resulting in modern twentieth (in the speech varieties this occurred in, the weak vowel in the final syllable of twenteth, twentith was clearly distinct from that in twenty); similar processes explain the modern pronunciation of thirtieth, fourtieth, etc. This analogy was assisted by synizetic pronunciations of words such as carrying, which allowed twenteth, twentith to be interpreted as contractions of twentieth.

来源：wiktionary