Wiktionary
why and wherefore
n. A full and complete explanation. Technically redundant, since "why" and "wherefore" are synonyms, but used to add emphasis. Also whys and wherefores.
Usage examples of "why and wherefore".
Curious to learn the why and wherefore, he allows himself to be drawn along the footpath out of sight.
He had a method of accepting things, without questioning the why and wherefore.
And there she told him why and wherefore, and how for his goodness King Mark slew him with his dagger afore mine own eyen.
Being entire strangers to the place, and seeing everybody running to and fro, they were much astonished, and having learned the why and wherefore, were curious to go see what was to be seen.