on thin ice
In a precarious or risky position, as in After failing the midterm, he was on thin ice with his math teacher. This metaphor is often rounded out as skate on thin ice, as in He knew he was skating on thin ice when he took his rent money with him to the racetrack. This idiom, which alludes to the danger that treading on thin ice will cause it to break, was first used figuratively by Ralph Waldo Emerson in his essay Prudence (1841): "In skating over thin ice our safety is in our speed."
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