"Why does she keep interrupting everybody?" Trump asked after Fiorina cut in over Sen. Rand Paul (R-Ky.), who was warning against a no-fly zone over Iraq. 
The crowd's brief laughter turned into a chorus of boos, as Paul asked whether he could finish his response. 
Trump has been criticized in the past for comments directed at women, particularly after a series of comments he made about Fox News host Megyn Kelly after the first GOP debate in August. More recently, he called Democratic National Committee Chairwoman Debbie Wasserman Schultz a "terrible person."
At the Fox Business Network debate in Milwaukee, Paul was warning that a no-fly zone would mean that America would have to commit to shooting down Russian jets. 
When he mentioned former President Ronald Reagan, Fiorina cut in to note that Reagan was willing to walk away from negotiating if he wasn't a position of strength.