froth at the mouth
1. Literally, to produce foam from one's mouth, as due to a disease or other ailment. All of a sudden she collapsed in a fit, convulsing and frothing at the mouth.
2. Figuratively, to be viciously and uncontrollably angry or upset. The protesters had formed outside the courthouse, frothing at the mouth as the alleged murderer made his way up the steps.
froth (or foam) at the mouth
be very angry. This phrase stems from the involuntary production of large amounts of saliva from the mouth during a seizure or fit.