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
Almost certainly it originated from pen and paper shorthand. The internet has a lot of history in regard to it potentially being due to T9 slang, but I remember it being shown to me before cellphones were in wide use.