ETYMOLOGY
probably from bump (to collide heavily or firmly, to knock) + -sy
EXAMPLE
“… How Tarlton landed at Cuckolds haven.
TArlton being one Sunday at Court all day, caused a paire of Oares to tend him, who at night called on him to be gone. Tarlton being a carousing, drunk so long to the Watermen, that one of them was bumpsie, and so indeede were all three for the most part: at last they left Greenwich, the Tide being at a great low fall the Watermen yet afraide of the Crosse Cables by the Lime-house, very dark and late as it was, landed Tarlton at Cuckolds-hauen, and said, the next day they would giue him a reason for it …”
From: Tarltons Jests Drawne into these three parts.
1 His court-witty iests.
2 His sound city iests.
3 His countrey pretty iests.
Full of delight, wit, and honest mirth.
By Richard Tarlton, 1611
Lexicophilia 