Green’s Dictionary of Slang

(US) the penis.

1834		‘Lustful Peg!’ in Libertine’s Songster in Spedding & Watt (eds) I 143: Why, this is my musket, my dear, / Which is always full charged when the ladies are near.
1837		‘The Chapter of Smutty Toasts’ in Icky-Wickey Songster 8: Here’s the soldier’s long musket.
1934		‘J.M. Hall’ Anecdota Americana II 97: Bonaparte’s musket / Was well under four.
c.1945		in R.G. Reisner Graffiti (1971) 108: Puritans with short muskets step up to the firing line.
1971		in R.G. Reisner Graffiti 108: Pilgrims with short muskets stand within firing range.