Green’s Dictionary of Slang

a sexually active person.

1947		C. Willingham End as a Man (1952) 126: Old man McKee used to get his share, too. He was the rootinest old fellow I ever saw, and I saw some real rooters back in those days.
1988		McGill Dict. of Kiwi Sl. 94/1: rooter active heterosexual.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].