The full form of WDYM is What do you mean
“WDYM” stands for “What do you mean”.

So think about it. Pretend you’re texting someone and ask “what does “WDYM” mean?” They’ll probably respond with “what do you mean?” In which case, you’ll have to explain that you want to know what the letters “WDYM” stand for? If the responder is anything like the people I talk to, they’ll come back with something like, “what do you mean” instead of writing out that “WDYM” means “What do you mean?”

“WDYM” = “W” (What) + “D” (Do) + “Y” (You) + “M” (Mean)