The Gnomish Lexicon of the 1910s had odinthos “seventeen”, originally a combination of G. odin “seven” and G. sath “ten”, but then the ns > nth, after which the final th dissimilated to s: odinsath > odinthath > odinthas (GG/13). Then at some point the a in the final syllable became o, producing a suffix -thos also seen in other Gnomish “teen” words.