be (walking) on thin ice

To be doing something very risky or dangerous that could result in imminent disaster or ruin. I hope you realize that you'll be on thin ice if you decide to gamble your employees' retirement funds on such a dodgy investment. You're walking on thin ice by continuing to come in late. If the boss notices, you'll be fired. After his recent controversial comments, the senator is walking on thin ice with his constituents.

A hazardous course of action or conversation. “In skating over thin ice our safety is in our speed,” wrote Ralph Waldo Emerson in his essay Prudence (1841). Literally, it is possible to skate over very thin ice without breaking through if one glides quickly enough, but Emerson was already using the expression figuratively.