Green’s Dictionary of Slang

mentally unbalanced, insane.

1838–9		Dickens Nicholas Nickleby (1982) 439: ‘He was a little wanting here,’ touching his forehead.
1911		Times 12 July 11/3: Making him seem to be what village-people call ‘wanting’ .
1924		J. Buchan Three Hostages in Buchan (1995) 74: Had something given in my brain last night [...] so that now I was what people call ‘wanting’?
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 199: A person who is ‘wanting in the upper storey’ is: bats.
1976		‘J. Bell’ Trouble in Hunter Ward 180: Maisie, poor mite, wanting from birth, she was [OED].