Pretending to be something you aren’t, to be a hypocrite. We all know the
origins of this one from movie swashbucklers, where the pirate ship at the
moment of attack lowers its friendly or neutral flag and hoists the deadly skull
and crossbones. The unwritten law of the sea, of course, required that all ships
display their true flags or colors so that they could be recognized as friend or
foe, but the Jolly Roger was by its nature exempt.