twentieth

Origin

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.