As with 'awesome sauce', this is an exclamation or reserved sign of approval. The difference in this case being that the exclamation comes in the form of both complimenting and envying the guy wearing a Green Jacket.
Steve: So'd anything happen with that chick you met at the bar last weekend?
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 $2million.