-th 1
a suffix forming nouns of action ( birth ) or abstract nouns denoting quality or condition ( depth; length; warmth ).
[ ME -th ( e ), OE -thu, -tho, -th (var. -t after a velar, f, or s ); c. Goth -itha, L -tus, Gk -tos ]
-th 2
a suffix used in the formation of ordinal numbers ( fourth, tenth ), in some cases, added to altered stems of the cardinal ( fifth; twelfth ).
[ ME -the, -te, OE -tha, -the (var. -ta after f or s ); c. ON -thi, -di, L -tus, Gk -tos; see -ETH ]
-th 3
var. of -eth 1 : doth.