Wordorigins.org

This term for a floozie or loose woman, especially one of low intelligence, is from the Italian bambino, or baby. It makes its English appearance in 1918, in Rosano’s Price of Honor in a bit of Italian-accented dialogue:

She flop! An’ il bimbo he break da boni.

The original meaning was a stupid, inconsequential, or contemptible person. By 1920 the floozie sense had developed. From Zeidman’s Burlesque (1920):

This Dix bimbo is a dangerous woman...a sassy girl with...more than a figure—a physique.

(Source: Historical Dictionary of American Slang)