"This trip is a blast, brother, but it's been a week and I'm starting to miss my ol' lady."
"Don't worry about her, man. She's at home, relaxing... I'm sure Sancho's taking real good care of her."
"Who's Sancho?"
"He's like a Mexican Jody. You know.."
"Aw, fuck you man. You're such a jerk. I'm calling her..."
Jody is the bane of soldiers, touring musicians, travelling salesmen, and prisoners everywhere. He's been around as long as men have been leaving home for extended periods of time. The term "Jody" is a generalized label for a man who has sexual relations with another man's wife or girlfriend while the husband or boyfriend is away. He's there to comfort her when you can't be. He keeps her company, keeps her loneliness at bay. Sometimes keeps her affections even when you get back from wherever you've been off to.
The term has its origins in American blues music. "Joe De Grinder" was a character of myth in the blues tradition whose role in storytelling was to cuckold men and steal their women; to provide a little conflict. During WWII, this concept was carried to war by African Amercian soldiers. The language spread to popular usage in such a fitting context. It was gradually abbreviated to "Joe D." and eventually "Jody." There were even military marching songs about Jody. According to Cordelia's writeup above, even marching songs that weren't about Jody eventually came to be known as Jodies. Crazy!
Ain’t no use in going home
Jody’s got your girl and gone
Gonna get a three-day pass
Just to kick old Jody’s ass.
— U.S. Army marching cadence, circa 1944
The same concept is expressed in Mexican culture as "Sancho." Have a look at the lyrics to Sublime's hit song, "Santeria" for appropriate context.