mall ninja

Etymology
Originated in several 2000-2001 discussions on Internet forums devoted to firearms, in which a poster bragged about his firearm expertise while claiming to be a "Sergeant of a three-man Rapid Tactical Force" at an American mall, trained in "ninjitsu". He was soon nicknamed "the mall ninja".

Noun

 * 1)  A person who brags about being a weaponry expert and connoisseur, but exposes their ignorance by making improbable claims about their experience or skill.