Shet is the name of a Hardy Boys character. Madmad and Eric have taken to playing, "Hardy Boys." Imagine hearing this over and over again...
"Shet...are you there?"
"Shet...did you get the rope?"
"Shet...help me get him!"
"Shet...he's got a gun!"
"Shet...how are we going to get away?!"
"Shet...NO! Not that Shet!"
You may have to actually say it outloud and then just watch whoever is close to you to see if they give you a look that says, Uh...did that really just come out of your mouth?