twelve + -ty, from Old English hundtwelftiġ. Compare hundseofontiġ, hundeahtatig, hundniġontiġ and hundtēontiġ for modern seventy, eighty, ninety and one hundred, reflecting the old Germanic hybrid base-ten and base-twelve numbering system. This hund was used for counting higher decades, and eventually became the root of hundred. Hund originally meant 120 (now called a long hundred or great hundred, and preserved in the term hundredweight), but was used to translate multiples of Latin centum (“100”), and eventually picked up that value when used in isolation as well.
- (nonstandard) One hundred (and) twenty; 120.
- 1854, Thomas H. Palmer, Arithmetic, Oral and Written, page 36:
- Thirty-six from a hundred and twenty-nine? [twelvety-nine.]
- 1922, International Ladies' Garment Workers' Union, Report and Proceedings of the Convention, page 1:
- The twelfty-six  weeks' strike taught us the value of being organized
- 2004, J. Michael Straczynski, The Lurker's Guide to Babylon 5: The Quality of Mercy:
- Minbari use base 11, not base 10, so twelve would be eleventy-first year, and so on. Minbari base eleven includes fingers and head, from which the principle of mathematics comes. […] Eleventy-seven = Eighteen base ten. One, two, three, four, five, six, seven, eight, nine, ten, eleven. Eleventy-one, eleventy-two, eleventy-three, eleventy-four, eleventy-five, eleventy-six, eleventy-seven, eleventy-eight, eleventy-nine, eleventy-ten, twelfty. Twelfty-one, twelfty-two, twelfty-three, twelfty-four, twelfty-five, twelfty-six, twelfty-seven, twelfty-eight, twelfty-nine, twelfty-ten. And so on. Who here still has a problem with this?