Sentence:  Trekkers knew that if a "<WSD>red shirt</WSD>" — a non-regular cast member garbed in a red Starfleet tunic — was assigned to a landing party with Captain Kirk, the odds were very much against him or her returning from that mission. "<WSD>Red shirts</WSD>" were the expendable members of the crew, and viewers were never really surprised if the poor innocents were snuffed by the episode's end. 
Meaning of the word marked with <WSD> tags:  A minor character or member of a group, especially one who is expendable or likely to be killed.