The number 45, or a firearm chambered in .45 caliber, commonly referring to a .45 ACP (Automatic Colt Pistol).
Compound of 'forty' (Old English 'feowertig') and 'five' (Old English 'fif'). The gun reference comes from the cartridge designation '.45', which became slang in 20th-century American English.
The .45 ACP became iconic through American military history—adopted by the U.S. Army in 1911 and used extensively in both world wars—so the term 'fortyfive' became slang partly because of its legendary stopping power and cultural prominence in crime fiction and hip-hop!
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