A euphemistic metaphor for a
woman's vaginal area when she is on her back and her thighs are
spread (in ready-to-fuck position) to form a shape resembling the curvature of the surface of an
equestrian saddle.
I was on the outs with my girlfriend for a while, but now I'm back in the saddle again.
A good example in
music is the sexy 1980's country
music hit by Mel Tillis, "I Got the Horse (i.e. penis) and She Got the Saddle (i.e. pussy)", with the chorus line, "together we're gonna ride, ride, ride!"
Another: In the movie Semi-Tough, when Kris Kristofferson finally gets the
woman he loves, Jill Clayburgh, back in bed and is ready to fuck her (you can see his big hard-on poking up the sheets) the background
music starts playing Gene Autrey's 1940's cowboy song, "I'm Back in the Saddle Again", as the scene fades.