A loser or tool. Someone who is not a main player in a social situation; an extra. Reference from Star Trek; similar to the terms "red shirt"--the generic term for the "other" guy that gets beamed down to the planet with Captain Kirk, Mr. Spock and Dr. McCoy to "see if the aliens are hostile". Invariably, the the aliens are hostile, and Yeoman Johnson dies. The term also refers to people in a social group who are "extras". For example, someone's current boyfriend who's destined to soon get kicked to the curb or is just a pawn in the game. The term is unflattering and dismissive.
"Is Sally still engaged to her Yeoman Johnson? But she bangs everyone at the office!..that kid is such a chump!".