The placing of one's fist into one's grundle and letting it bake there for a minute, then removing it and punching another person across the face with it.
Bitch, if you don't shut your mouth, you're gonna get a big ol' grundlemuffin!
A Shackteau is a humble, weather-beaten, structurally questionable shelter located in a spectacular or highly coveted place—Wales, Jackson Hole, Sun Valley, Crested Butte, coastal Maine, the Alps—where the building itself may be worth almost nothing, but the dirt, view, access, and mythology make it absurdly valuable.
In use:
Shackteâu - We thought it was an abandoned shed until the realtor called it a rare alpine Shackteâu with unobstructed views and listed it for $2 million.