A/W means Autumn/Winter
The Oxford dictionary recently adopted a slew of new words. A/W is among them. Here's a brief overview of the word:
A/W, according to the new definition means "autumn/winter (denoting to fashion designed for the autumn and winter seasons of a particular year)." It's not abundantly clear why something that means "autumn/winter" has to be related to fashion. I say A/W can go anywhere seasonal words can go, but then again, I'm just a dude and this is the Oxford dictionary. If you want to dive more down that rabbit hole, you ought to check out this David Foster Wallace classic: Authority and American Usage.
Popular Alternative 1
Now, A/W doesn't necessarily mean this, however, since most people don't run in fashion circles where this kind of thing is relevant. In fact, the most common use of A/W is to refer to the hamburger, hot dog, and root beer fast food chain known more correctly as A&W.
Popular(?) Alternative 2
According to this website, A/W is internet slang for "anyway." There may be some validity to this as indicated by the existence of this UrbanDictionary page, but in any case, this doesn't appear to be a very popular use of the term, at all.
Here's a picture with all three usages of the word: