Slang for being horny or turned on. Also used in the sense when drunk. Was invented during the late 40's
Timmy: Hey John do you see that girl
John: Yeah she's so on Turning
Timmy: I was thinking the same thing
John: Look here she comes....
Some one who is sexy beyond belief. They turn you on.
MMMMM!! Have you seen Jesse in that shirt. Talk about on turning!