1. artillery designed to shoot upward at airplanes

Synonym: antiaircraftantiaircraft gunflakpom-pomack-ackack-ack gun

2. intense adverse criticism

e.g. Clinton directed his fire at the Republican Party
the government has come under attack
don't give me any flak

Synonym: fireattackflakblast

3. a slick spokesperson who can turn any criticism to the advantage of their employer

Synonym: flak catcherflakflack catcher