red·shirt (rěd'shûrt') tr.v.
red·shirt·ed, red·shirt·ing, red·shirts To keep (a college or school athlete) out of varsity competition for one year in order to extend the athlete's period of eligibility.
[From the red jerseys worn by such athletes to distinguish them from the regular players.] red'shirt' adj. & n.
"to withdraw (a player) from the varsity team to add a year to his or her eligibility," 1955, in allusion to the red shirts worn by athletes on the scrimmage squad.