Slow Burn Meaning

(idiomatic) A gradually increasing feeling of anger or frustration.

Example: 1957 July 1, "Scoreboard," Time:
  In the ninth inning of a game with the Pittsburgh Pirates, Johnny Temple, Cincinnati Redleg second baseman, let a hot grounder sizzle through his legs, looked up to see the Scoreboard flash "error" and began a slow burn.
2008 July 25, Ty Burr, "‘Step Brothers’ is crude, rude—and funny (film review)," Boston Globe (retrieved 8 Apr. 2009):
  The comedy comes from the patient slow burn of the parents as they try to ignore the explosive belligerence of the boys.