Where does “hence why” come from?

hence why (English) comes from English hence, from Middle English hennes, from Old English heonan, from Old English hē, from Proto-West Germanic *hiʀ, from Proto-Germanic hīz — nominative plural masculine of *hiz; these, these...

hence why (English): For this reason, as a result; therefore, that's why

Definitions

  1. For this reason, as a result; therefore, that's why

Ancestry of “hence why”, step by step

hence why traces back through two separate lines of ancestry.

via English hence

StepLanguageWordMeaning
1Englishhencefrom here, from this place, away; from the living...
2Middle Englishhennes
3Old Englishheonanfrom here, hence
4Old Englishhey
5Proto-West Germanic*hiʀ
6Proto-Germanichīznominative plural masculine of *hiz; these, these...

via English that's why

StepLanguageWordMeaning
1Englishthat's whya statement to emphasise the reason or explanation just given
Every word from Proto-Germanic hīz