The "grim trigger" strategy is an angle in game theory, particularly pertaining to infinite iteration and the prisoner's dilemma. The idea is to create a Nash Equilibrium (NE) of the Pareto Optimal solution through repeating games where there was previously a non-Pareto optimal dominant strategy.

...a little payoff matrix to refresh you
2,2   0,7
7,0   1,1

The strategy basically goes like this: "Do not confess (cooperate with me) and we will continue a cooperative tradition to perpetuity. The payoff for this is 1+1*delta+1*delta^2+...1*delta^n where delta is your discount rate for future payoffs. (remember the lowest score is best in prisoner's dilemma) If you ever confess (cross me) then I will forever punish you by confessing. Period. No forgiveness possible. Assuming you find this to be a credible threat (a fair assumption unless you have some other information about my personal discount rate) it is an NE for you to cooperate.