Green’s Dictionary of Slang

imbo n.

[SE imbecile + -o sfx (3)]

(Aus.) a fool, a simpleton.

1950, 1955		‘No. 35’ Argot in G. Simes DAUS (1993).
1953		Baker Aus. Speaks.
1974		J. McNeil How Does Your Garden Grow Act III: I reckon. Top idiots. Imbos.