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To go off half-cocked or in a willy nilly sort way. A combination of barreling through something and monkeying around.

Most commonly used in an MMORPG while dungeon crawling with a large group whose members who may be apt to act like Leeroy Jenkins and get everyone killed.
"Ok when we get in to Blackwing Lair just stay put and don't go barrel monkeying around in there."
by Harvey Lopez January 10, 2007