(adj.) Insane, particularly if characterised by exaggerated or cartoonish behaviour ; gibbering in an nonsensical fashion. (After RogerIrrelevant in Viz magazine.)
He stood in the corner shouting, "Wibble! Wibble! More beetroot, Archdeacon?" I'm afraid he's gone completely hatstand.
Possibly originating from a man who kept trying to put his wife on a hatstand because he thought she was a hat. Chronicles by Oliver Sacks in his book “The man who thought his wife was a hat”.
Hatstand means crazy.