When out at a club or a party, being on the lookout for that one douchebag you want to avoid at all costs because he always latches onto you and your friends like a leech and basically ruins the fun for everyone. Because the person or persons on bag watch are essentially providing a life saving service to the rest of the group, they must have eyes like a hawk and be as stealthy as a ninja.
Todd: "Hey Rabbit, that party tonight at Kelly's should be fuckin' awesome. I just hope that Chris don't show up."
Rabbit: "Don't worry, dude. Matt and I are taking turns on bag watch, so if we see him, we will give everyone a heads up and we can all bail."
Todd: "Fuck yeah. With you guys on the case we definitely have no worries".
Blowing the bullshit whistle is method of calling someone out for not telling the truth about something. When a person knows a statement or story is total bullshit, he will make a “tweet-tweet” sound like a steam whistle. It’s especially handy in group settings like a smoking circle where the embarrassment for the liar can be maximized.
Todd: “Yo’, Chris. I heard you and Rabbit went to the club last night. How was it?”
Chris: “Dude, it was crazy. The bitches wouldn’t leave me alone. I guess they all wanted my potato dick.”
Rabbit: (exhaling an especially big bong rip) “Tweet-tweet. I'm totally blowing the bullshit whistle on that.”