Also ‑eth.
Forming ordinal and fractional numbers.
Old English ‑(o)tha.
The ending is added to numbers from four onwards to make ordinal numbers that define a thing's position in a series: fourth, fifth, eighth, twenty-ninth, hundredth. When the number ends in y, that letter changes to i and the ending becomes ‑eth: twentieth, fiftieth, ninetieth. The same form is used in fractions: two fifths, three eighths. See also ‑teen (for ‑teenth).
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