Unintentionally or accidentally getting an erection. This usually occurs during inopportune moments such as during a business meeting or when guys (gay or straight) are playing sports together and make physical contact with one another. It is often considered to be embarrassing to everyone involved. It most often happens to guys when they are going through puberty because hormones are raging out of control and erections happen even when the guy is not thinking about anything sexual.
Guys in locker room after showering after a football game:
Tony: "Dude, are you gay? Everyone noticed you had a boner in the shower."
Derek: "No, man, I couldn't help it. It was just unintentional stem."