The story
* ten + -ty Perhaps from Old English hundtēontiġ (compare hundseofontiġ, hundeahtatiġ, hundniġontiġ, hundendleftiġ, and hundtwelftiġ for modern seventy, eighty, ninety, and nonstandard eleventy, twelfty, reflecting the old Germanic hybrid base-ten and base-twelve numbering system); perhaps reconstructed based on eleventy etc. and -teen.
Etymology from Wiktionary, CC BY-SA 4.0.