A "lesnar" is an individual who seeks to elevate their social status by exaggerating their credentials or by acting to others as something they are not. This tends to be in homage of the famous wrestler Brock Lesnar.
Usually said, on its own, as a sentence by itself, expressing extreme puzzlement at an excessively underwhelming situation or outcome. This was pretty much everyone's reaction to Brock Lesnar beating the Undertaker's Wrestlemaniawinning streak at Wrestlemania XXX.
When you wake up in the morning with a puffy face and it's hard to open your eyes after a couple of hours of sleep or a heavy night of drinking, making you look like Brock Lesnar after he landed on his face after botching a Shooting Star Press at Wrestlemania XIX.