Stands for "What would Larry do?", referring to Larry David, "Seinfeld" creator and star of the HBO series "Curb Your Enthusiasm". It's usually thought of when one is in an awkward situation and not sure of what to do, but Larry would do something ridiculous and get in trouble for it most of the time.
I was in a restaurant with my wife and another couple, and when the bill was about the come the guy from the other couple "conveniently" had to leave to go use the bathroom, leaving me to pay for the dinner. So I thought to myself "WWLD", and when he got back I confronted him about how I know about "the bathroom trick!".