Green’s Dictionary of Slang

1. (US) a foolish, unimportant person.

1913		J. London Valley of the Moon (1914) 321: ‘Bosco. Remember him?’ ‘He eats ’em alive! He eats ’em alive! Bosco! Bosco!’ Saxon responded, mimicking the cry of a side-show barker.
1925		Amer. Legion Weekly 5 June 7: Any Broadway bosco will tell you [HDAS].
1938		J. Archibald ‘Bird Cagey’ in Popular Detective Jan. 🌐 Jeepers could have bumped off Drupe. The bosco had a reason.
1940		J. Archibald ‘Short Order Crook’ in Ten Detective Aces Apr. 🌐 I bet he is a long-faced bosco with a purplish bugle.

2. a ferocious madman.

1914

Ade ‘The New Fable of The Toilsome Ascent’ in Ade’s Fables 183: As for Wall Street and the Plunder-bund, when he got after them, he was a raving Bosco. A regular Woof-Woofer and bite their heads off.