the antiquity of this word dates back all the way back...waaay back to the era when the young and tight crustations were forming into the alien robots which took the divinely noun-heavy novels from the future and burned them so that they and their family friends whom kill devoid of compunction can enjoy the fine eats at Maggie's on 42nd.
m/w can be often spotted in a bio, typically on discord or twitter with a name or @ usually followed after. it stands for "matching with" which indicates the context between a pfp to show who has the other half of the picture.
Person 1: Hey, what's your pfp?
Person 2: Oh, I'm just m/w my friend John. He wanted to match profile pictures so I agreed.