The term is derived from a legendary soldier in the online PC game World Of Warcraft in which Leeroy Jenkins interrupts his comrades during a moment of strategizing on the outset of an imminent battle to which their odds of survival are 32.33% and screams his now famous battle cry, "LEEEEEEERRRROOOYYY JENKINS!" and charges into enemy territory. Leeroy's spontaneous outburst and act of bravery shocks and inspires his fellow soldiers and they too charge after Leeroy. The battle ultimately ends in a decisive defeat leaving no survivers thus immortalizing Leeroy Jenkins as the embodiment of bravery and sacrifice.
The courageous act was recorded and posted on youtube in 2006. just type Leeroy Jenkins into the search field.
The event has been referenced on many flatforms and mediums such as the popular television show Final Jeopardy and also on The Howard Stern Show.
"we got our asses kicked the other night at the bar. The whole goddamned thing was a Leeroy Jenkins."
Or used as a verb, example: "I will Leroy Jenkins your ass!"