pretending that you don't want anything from McDonald's when, in reality, you do.
Often, someone who is McBluffin' will refuse to order anything from the drive thru, then subsequently ask for "a bite" of whatever it is you are eating, and steal all of your fries. McBluffers should be called out before ordering so they are forced to fess' up.
*car pulls up to the ordering window*
McDonald's Intercom: Hi, welcome to McDonald's may I take your order?
John (to intercom): Yep, Just one moment.
John (to Megan): Hey Megan, what do you want?
Megan: I'm actually not hungry, nothing for me thanks
John: Pshhht whatever Megan you better order something I know you're McBluffin'! Last time you "didn't want anything" you ate all my sausage McMuffin!!!
Megan: ...fine I'll have a Big Mac meal
John (to intercom): Yeah I'll have one Big Mac meal, a 6-pack of chicken nuggets....