Green’s Dictionary of Slang

rooter n.5

[root v. (5a)]

a sexually active person.

[US]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.
[NZ]McGill Dict. of Kiwi Sl. 94/1: rooter active heterosexual.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].