Similar to No offense. It's usually said by a person after they've insulted you and meant everything they said, but don't want to upset you or get punched in the face as a result.
Mike: "Dude your a fucking faggot and no ones likes you! ...Haha only joking :)"
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.