mortal lock
n.— Note: A cinch, an odds-on favorite, a guaranteed thing or event. «It is a sure bet—a mortal lock as they say in the business— that if Robespierre was around today, we would be standing knee-deep in decapitations.» —“Five Things: Who Will Punish the Bankers?” by Kevin Depew Minyanville (New York) Apr. 6, 2009. (source: Double-Tongued Dictionary)
Further reading
If someone’s extremely annoyed or frustrated, you describe them with the idiomatic expression they’re fit to be tied. But where did this saying come from? This is part of a complete episode.
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