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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.

Why are these lesnars spitting bars in the canteen?
by Dougza March 02, 2011
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 Wrestlemania winning streak at Wrestlemania XXX.
"Lesnar? Lesnar. Seriously?!? Lesnar..."
by RioDragon April 06, 2014