A character in a science fiction or adventure story whose sole dramatic purpose is to get killed by the story's villain and/or intinerant monster.

Taken from the propensity of security officers on the original Star Trek series (who typically wore red uniform tops) to be killed in the epsiodes' pre-opening-credits teasers.

A term used to reference cannabis/marijuana/weed.

The person most likely to die first in any TV show, or film, regardless of what colour sweater they're wearing.

Taken from Star Trek the Original Series, where the security guards wore red, one always 'beamed down' from the spaceship to planets on exploratory missions and always died first. Always.
Don't ever wear a red shirt on a space mission!

Expendable characters. Usually say one line or less before being killed in a plot-convenient manner.
Most often seen in classic 60's Trek.

a term used in American college athletics for players, usually freshmen, withheld from competition in order to delay the beginning of their 4-year eligibilty period

A play on college sports' "Redshirt" rule: they can't play, but they're still a part of the team.
Redshirt is another name that refers to underage girls (typically between the ages 16-17).
They're "a part of the team" = you can check them out, but "they cannot play" = you can't have sex with them because that's illegal.

In star trek episodes,the expendible crew member who always dies wears a red shirt .so that it has come to mean the failure or defeat of an idea or action.