-th1

1.
a suffix forming nouns of action (birth) or abstract nouns denoting quality or condition (depth; length; warmth).
Origin
Middle English -th(e), Old English -thu, -tho, -th (variant -t after a velar, f, or s); cognate with Gothic -itha, Latin -tus, Greek -tos

-th2

1.
a suffix used in the formation of ordinal numbers (fourth, tenth), in some cases, added to altered stems of the cardinal (fifth; twelfth).
Origin
Middle English -the, -te, Old English -tha, -the (variant -ta after f or s); cognate with Old Norse -thi, -di, Latin -tus, Greek -tos; see -eth

-th3

1.
variant of -eth1 :
doth.
British Dictionary definitions for -th

-eth2

suffix
1.
a variant of -th2 twentieth

-th1

suffix
1.
(from verbs) indicating an action or its consequence: growth
2.
(from adjectives) indicating a quality: width
Word Origin
from Old English -thu, -tho

-th2

suffix
1.
forming ordinal numbers: fourth, thousandth
Word Origin
from Old English -(o)tha, -(o)the
Word Origin and History for -th

word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha-, from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins] (cf. Sanskrit thah, Greek -tos, Latin -tus).

suffix forming nouns (e.g. depth, strength, truth, etc.), from Old English -ðu, -ð, from Proto-Germanic *-itho, abstract noun suffix, from PIE *-ita (cf. Sanskrit -tati-; Greek -tet-; Latin -tati-, as in libertatem "liberty" from liber "free").