A phrase most commonly used in a modification of the game Half-Life called Natural-Selection (www.natural-selection.org). Undoubtedly a purposeful misspelling, a shirtgun is actually referring to the shotgun contained within in the game. The term "shirtgun" was was found (or created, either is possible) and used usually in combination with other misspellings.
Omgz!11oneone Tehre is a lork on teh clorf! Commandar! We need teh shirtguns!