Where does “one-hundred-thousandth” come from?

one-hundred-thousandth (English) comes from English -th, from Middle English -eth, from Old English -eþ, from Proto-Germanic -þi, from Proto-Indo-European -ti.

Ancestry of “one-hundred-thousandth”, step by step

Step	Language	Word	Meaning
1	English	-th	Used to form nouns from verbs of action; Used to...
2	Middle English	-eth	Used to form the third-person singular present...
3	Old English	-eþ	Third person singular present indicative ending of class 1 weak verbs
4	Proto-Germanic	-þi	—
5	Proto-Indo-European	-ti	—

Every word from Proto-Indo-European -ti →Every word from English -th →Every word from Middle English -eth →