"I am sorry that I said it out loud," Lightfoot said.
The mayor made the remark as the first-vice president of Chicago's Fraternal Order of Police walked up to speak at Wednesday's City Council meeting.
Patrick Murray was disputing the recent firing of four officers over the Laquan McDonald investigation.
The Fraternal Order of Police released a statement, saying "Mayor Lightfoot's contemptuous remark is a misguided and dangerous thing to say to a 30-year veteran police officer and FOP representative... particularly at a time when the city is facing such chronic violent crime."