-th1 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).