Where does “itsself” come from?

itsself (English) comes from English its, from English it’s, from English -’s, from Middle English -s, from Old English -es, from Proto-Germanic -as, from Proto-Indo-European -ēs, from Proto-Indo-European -ess — Creates action nouns or result nouns from verbs;...

Ancestry of “itsself”, step by step

itsself traces back through two separate lines of ancestry.

via English its

StepLanguageWordMeaning
1EnglishitsBelonging to it; The one belonging to it; plural...
2Englishit’sThere's, there is; there're, there are
3English-’sAttached to a noun or noun phrase linked to a genitive (Compare of mine, etc.)
4Middle English-sAdverbial genitive ending, developed into the...
5Old English-espossessive marker, indicating that an object...
6Proto-Germanic-asused in formation of adverbs
7Proto-Indo-European-ēsPrimarily forms adjectival bahuvrīhi compounds
8Proto-Indo-European-ess
9Proto-Indo-European-osCreates action nouns or result nouns from verbs;...

via English Self

StepLanguageWordMeaning
1EnglishSelfHimself, herself, itself, themself, themselves; that specific (person mentioned)
2Middle EnglishsalveAlternative form of sauf
3Old Englishsealfsalve
4Proto-West Germanicsalbusalve, ointment
5Proto-Germanicsalbōsalve, ointment
6Proto-Indo-Europeansolp-éh₂
7Proto-Indo-Europeanselp-fat, oil
Every word from Proto-Indo-European -osEvery word from Old English -esEvery word from English -’s