1. a metaphor used to call another's bluff.
I'm gonna have to call the bullshit flag on that sea story.
When someone is talking out of their ass, they may sound like they are telling the truth but you can clearly tell they are lying.
Dude, this guy was telling me that he has visited 10 countries in 30 days, so I raised the bullshit flag on this one and called him out! he turned so red,you should have seen his face! haha