Fuck or Die is a fanfiction trope in which the author puts two or more characters into a situation that forces intimacy between them.
Like many tropes it can be and often is combined with others such as sex pollen or Aliens Made Them Do It and Virginity Sacrifice. It can result in dub-con or non-con fanworks.
Original Fannish Source
Although Fuck or Die has been used as a plot device in many fandoms since, its starting place was probably Spock's Pon Farr, where, in canon, Spock enters Plak Tow and must fight, fuck or die. Many K/S stories started with Spock going into pon farr, and as Trek fans moved on to other fandoms, they took the trope with them.