@kekkaishi said in Why people use the phrase "w/" to mean "with"?:
@bi4smooth Thanks for the explanation.I genuinely believe that people use it because it's trendy, not because it's easier
Well, not to "pop your bubble" on this one, but when I mentioned it's use by my mother - it was 40+ years ago, and it was while being written longhand, not on a keyboard (much less a phone or tablet!) - so while it may be a "trendy" item today, if so it's a re-trend (or a borrowed trend?) 
Truth be told, the "w/" was not written as unique, separate characters, it was all one stroke (I've looked online for an image, but haven't found one)... imagine writing a "w", but taking the upper-right of the w, moving the pen up and to the right very slightly, then striking a hard downward slash...
Similarly, something close to a "+" was used for "and" : imagine a plus character, with the "south" corner connected (by a curve) to the "west" corner... or, start with your pen at North, strike down, straight, to South, curve up and to the left to West, then strike straight to the right to East...
Also, w/f meant "wherefore" (a common legal tern), b/c = "because", b/f = "before", w/h = "withheld", w/f = "wherefore" (another common legal term)... and there were MANY others....
A favorite was "fu" -- which meant follow-up! 
I was in my teens, and this is all from memory... there HAS to be an Internet resource for these - but I couldn't find one! My mother is a smart lady, but she didn't "invent" this shorthand! (And I don't mean the "secretarial shorthand" that is a whole other "language"!)