Sex Slang (2007) by Tom Dalzell and Terry Victor says:
booty; bootie noun
1. the buttocks US, 1928
- He's Cyndi Lauper's boyfriend, so no skin search. Cyndi ouldn't want us looking up his boodie. - James Elroy, Suicide Hill 1986
2. the vagina US, 1925
- I've got a body as well as a booty. - Partlet, Booty Snatchers 1979
(The Concise New Partridge Dictionary of Slang and Unconventional English (2007) by Eric Partridge, Tom Dalzell and Terry Victor gives a subset of this same definition.)
The OED expands on these same two definitions, but with a wider gap in earliest use:
1. Sexual intercourse; a person (esp. a woman) regarded as an object of sexual ambition or desire. Also (occas.): the female genitals. Cf. ass n.2 1b [sexual gratification. Also, a woman or women, regarded as an object affording this.].
1926 C. Van Vechten Nigger Heaven ii. iii. 215 Now..that you've gone white, do you really want..pinks for boody?
2. The buttocks.
Prob. the earlier sense, esp. given the similar sense development of ass n.2, pussy n. 3, etc.
1959 F. L. Brown Trumbull Park 363 Getting kicked in the booty would be mighty discouraging too.
Their etymology is:
Probably an altered form of botty n.; compare batty n.2 Perhaps influenced (especially in sense 1) by booty n.1 [Plunder, gain, etc.]