Green’s Dictionary of Slang

(US) a posing incompetent, esp. on stage.

1926		Variety 29 Dec. 7: A ‘milkman’ was a real hambo, who stole more bows than the applause warranted at the finish of his act [HDAS].
1932		W. Winchell On Broadway 8 Apr. [synd. col.] He was introduced to a hambo in Hollywood as ‘an author’.
1946		R.L. Bellem ‘Latin Blood’ in Speed Detective Aug. 🌐 One hambo spanks the other across the mush with a glove; challenges him to a duel.
1952		W. Winchell ‘On Broadway’ 26 Aug. [synd. col.] Then there’s the hambo who once squawked [...] that he was given a minor role.