Hi, English isn’t my mother tongue so I was asking myself that question since I first encounted a w/… Back then I was like: “What tf does ‘w slash’ stand for?” And when I found out I was like “How, why, and is it any intuitive?” But I never dared to ask that until now
Mathematicians didn’t want to write a conclusion paragraph to their proofs to say that that result proves what they wanted to show, so they just put “QED” at the end which is some Latin/Greek phrase to the effect.
Then they got too lazy to write that because they do proofs all the time, so they just put a box ∎ that means “so, basically yeah. there you have it.”
Q.E.D. is “quod erat demonstrandum”, meaning “thus, it has been demonstrated”.