Green’s Dictionary of Slang

the penis.

1664		C. Cotton Virgil Travestie (1765) Bk I 29: He took his Mother for a Maid; / A great Mistake in her whose Bum / So oft had been God Mars his Drum.
1675		C. Cotton Scoffer Scoff’d (1765) 238: He beat (oh, never such a Drum!) / A Point of War upon her Bum.