How did the term "bimbo" referring to a dumb person come about?

This word derives itself from the Italian bimbo, derived from bambino a masculine-gender term that means (male) baby or very young (male) child (its feminine equivalent is bimba). Use of this term began in the United States as early as 1919, and was a slang word used to describe an unintelligent or brutish man.

It was not until the late 1920s that the term Bimbo began to be associated with females. The 1929 silent film Desert Nights describes a wealthy female crook as a bimbo and in The Broadway Melody, an angry Bessie Love calls a chorus girl a bimbo. The first use of its female meaning in the Oxford English Dictionary was in 1929, from the scholarly journal American Speech where the definition was given simply as "a woman".

https://en.wikipedia.org/wiki/Bimbo

The word bimbo did not derive from the Italian word bambino, though they sound similar. The word bimbo to describe an attractive female of low intelligence and loose morals derives directly from the actions of the Bimbo crime family. The Bimbo crime family operated for over 200 years as a gypsy family skilled in deceiving and extorting money from unsuspecting victims, then disappearing. The family hallmark was to use the uneducated but attractive family women to gain the affections of lonely male, then steal his money and suddenly leave. In the early 1900s the game escalated to include murdering their targets. It is then that the crime family became widely known throu associated newspaper coverage. For information supporting this thesis, reference the name Tene Bimbo, the king of the turn of the century gypsy Bimbo family.
In much the same way that a Bimbo woman would lure a victim, the term Bimbo came into use as a generalized reference to an attractive, uneducated and morally loose female who might seek a victim and take them for all they were worth.
There is no known correlation between the Italian word bambino, meaning a male baby, and the current meaning of a loose, attractive female woman being referred to as a bimbo. Interestingly, males of the Bimbo crime family also engaged in such scams, robbing unsuspecting widowers of their wealth.