The story
- Proto-Germanic · inherited from*tehundô
- Middle English · inherited fromtenth, tenthe
- English todaytenth
From Middle English tenth, tenthe. Old English had tēoþa (origin of Modern English tithe), but the force of analogy to the cardinal number "ten" caused Middle English speakers to recreate the regular ordinal and re-insert the nasal consonant. Ultimately from Proto-Germanic *tehundô. Equivalent to ten (numeral) + -th (suffix forming ordinals).
Etymology from Wiktionary, CC BY-SA 4.0.