Where does “that're” come from?

that're (English) comes from English ’re, from English are, from Middle English aren, from Old English earon, from Old English bēon, from Proto-Germanic bijōniz, from Proto-Germanic bijǭ, from Proto-Indo-European bʰey- — bee.

Ancestry of “that're”, step by step

that're traces back through two separate lines of ancestry.

via English ’re

StepLanguageWordMeaning
1English’reinflection of the verb "to be"
2Englisharesecond-person singular simple present of be;...
3Middle EnglisharenPlural present indicative form of been
4Old Englishearonthird-person plural simple present of wesan
5Old Englishbēonto be, become
6Proto-Germanicbijōniz
7Proto-Germanicbijǭbee
8Proto-Indo-Europeanbʰey-bee

via English þat

StepLanguageWordMeaning
1Englishþatso, so that, in order that
2Middle EnglishþatAlternative form of that
3Old Englishþætnominative/accusative neuter singular of se: the;...
4Proto-Germanicþatnominative/accusative singular neuter of *sa
5Proto-Indo-Europeantóddemonstrative pronoun; that; it
6Proto-Indo-Europeanto-it; that; the, that
7Proto-Indo-Europeanthis; that
Every word from Proto-Indo-European bʰey-