Where does “these've” come from?

these've (English) comes from English these, from Middle English þes, from Old English þēs — this.

Ancestry of “these've”, step by step

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

via English these

StepLanguageWordMeaning
1Englishtheseplural of this
2Middle Englishþesthese; Alternative form of þas; Alternative form...
3Old Englishþēsthis

via English ’ve

StepLanguageWordMeaning
1English’vein its sense marking the perfect or retrospective aspect
Every word from Old English þēs →Every word from Middle English þes →Every word from English these →