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 þēsEvery word from Middle English þesEvery word from English these