Where does “this've” come from?

this've (English) comes from English this, from Middle English þis, from Old English þis, from Proto-Germanic þat, from Proto-Indo-European tód, from Proto-Indo-European to-, from Proto-Indo-European só — this; that.

Ancestry of “this've”, step by step

this've traces back through two separate lines of ancestry.

via English this

StepLanguageWordMeaning
1EnglishthisThe (thing) here used in indicating something or someone nearby
2Middle Englishþisthis; Alternative form of þes
3Old Englishþisnominative/accusative neuter singular of þēs
4Proto-Germanicþatnominative/accusative singular neuter of *sa
5Proto-Indo-Europeantóddemonstrative pronoun; that; it
6Proto-Indo-Europeanto-it; that; the, that
7Proto-Indo-Europeanthis; that

via English ’ve

StepLanguageWordMeaning
1English’vein its sense marking the perfect or retrospective aspect
Every word from Proto-Indo-European