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1.
Any dumb or accidental action that causes an enormous let down, especially if it occurs at a critical time.

The term originates from sports, especially Rugby League, where "Coach Killers" are any dumb, accidental or glory-seeking plays occur which fall short of expectations. This term can be applied to any action which hopes or expectations were high, and one has the ability to adequately complete the task, however, the person manages to make a mistake, resulting in an unimpressive result.

In a Rugby League sense, a coach killer would be when a player, who could easily score by simply touching the ball on the ground, attempts to impress the crowd with a glory seeking dive, which results in the ball being dropped. This is especially true at critical moments of the game, such as when the scores are tied, or when scores are tight. The term originally referred to the player's coach, who, suffering a severe let-down, is metaphorically 'killed' by poorly executed play.
"Did ya get away with it?"

"Nah mate, old mate saw me and dobbed me in. Didn't think he'd do that"

"Bloody coach killer"
by rozza1 November 10, 2013