Green’s Dictionary of Slang

a fool of either sex; also as adj.

1978		L.K. Truscott IV Dress Gray (1979) 179: Hedges might be known among cadets as a dimbo. [Ibid.] 382: This dimbo major’s sitting there talking radio baby talk to his old lady.
1988		J. Milne Daddy’s Girl (1999) 18: Nobody becomes a private detective [...] nobody except a dimbo.
1999		Indep. Rev. 14 July 6: ‘When I saw the first script,’ Owen claimed, ‘I said, [...] believe me, I’ve done a few dimbos in my time, so I know gold when I see it.’.
2000		Guardian Editor 24 Mar. 7: Becks wasn’t such a dimbo.