Not the definition of the "girth" of the barrel, but rather it's bore - the width of the hole that goes through the barrel. Also, not coincidentally, the diameter of the bullet - 9mm.

Refers to a pistol, usually a semi-automatic, ostensibly a 9mm.

Aside: The U.S. Army's standard pistol is a 9mm semi-automatic.
by Rob Hruska February 06, 2004
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A semi-automatic handgun designed to fire cartridges in caliber 9mm Parabellum, aka: 9mm Luger, 9x19 NATO.
Dawgz ain't want'n nun o' mee, cuz muh nine'z got muh back!
by John Dixon January 13, 2004
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(1) The damn number that comes after eight, yet before ten.

(2) The not was well known, failed, system of measurement before the metric system.

I'm about to bury my foot nine inches into your ass.
by Stud McManly June 01, 2007
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n. used by punk ass gangstas, means a 9mm pistol, (one would be smart to run ones ass off when hearing this word being used by such types)
Best watch out fo' i whip out my niiine, an' BUST a cap
by Paddy C August 20, 2003
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