A character in fiction (usually TV or movies) that will meet their demise almost immediately at the beginning of the story. Can also be a real person that is clearly going to fail at the onset. Derived from the original Star Trek TV series in which unimportant characters (wearing red shirts) would accompany the main characters on an adventure and get killed before the first commercial break.
(pointing at TV screen) "See that ditzy chick with the radio? Red-shirt guy!"
"Manfred wants to play darts with them."