When someone or something randomly or is about to randomly and unknowingly put full effort, power, ability, etc. into something or somewhere when everything and/or everyone else isnt prepared, ready or unwilling to commit.
Based off the popularized WoW video where a WoW player named Leroy Jenkins charged into a dungon without letting or hearing out the group's plans of attack, instead charged in and shouted on voIP "LEEEEERRRROY nnnnnJJJJEEEEEENKINNSSSSSS"
The whole group went after his surprise attack, ending up in total devastation as the entire group was overwhelmed from aggro'ed monsters and being totally wiped out.
Look up Leroy Jenkins
Some uses of Going full Leeroy/Went full leeroy
"My car went full leeroy and now I dont have an engine."
"My computer didn't have the minimum power required, so my power supply died when my graphics card went full leeroy"
"I am about to go full leeroy if this light doesn't change to green."
"James decided to go full leeroy, now we are in jail; bail us out?"
"My Prius is going full leeroy, I cant stawp it!!1!"
"My cat is going full leeroy, he broke some shit now; can I call you back?"
"Nick went full leeroy thinking he could take him, now he is in the hospital."
"Here, hold my beer; I am going full leeroy."