The story
From Mike (“the letter M in the ICAO spelling alphabet”).
Etymology from Wiktionary, CC BY-SA 4.0.
40 mike-mike
Today it means: Any weapon firing a 40mm grenade. Often specifically the M203 grenade launcher, mounted underneath an M-16 or its variant. In the Marine Corps, often the Mark-19 Automatic 40mm Grenade Launcher. Full definition →