Where does “headless genitive” come from?

headless genitive (English) comes from English headless, from English -less, from Middle English -les, from Turkish -le, from Turkish ile, from Ottoman Turkish ایله, from Old Anatolian Turkish اِیلَه, from Proto-Turkic bile — with, together, also.

Ancestry of “headless genitive”, step by step

headless genitive traces back through two separate lines of ancestry.

via English headless

StepLanguageWordMeaning
1EnglishheadlessWithout a head; decapitated; Without leadership;...
2English-less(something); without (something). Added usually to a noun to form an adjective signifying a lack of that noun
3Middle English-lesLacking (something); without (something)
4Turkish-lewith, shortened form of ile; by, shortened form...
5Turkishilewith; and
6Ottoman Turkishایلهby
7Old Anatolian Turkishاِیلَه
8Proto-Turkicbilewith, together, also

via English genitive

StepLanguageWordMeaning
1EnglishgenitiveOf or pertaining to that case which expresses a...
Every word from Proto-Turkic bileEvery word from English -lessEvery word from Middle English -les