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.