Has many crude/colloquial uses but literally means "This Dick".
fr Italian/Neapolitan Stu Cazzo

Stu is the way Questo (The Italian word for 'this') is pronounced in the Neapolitan dialect.

Cazzo means dick/prick ( lit. penis)

btw - It does not mean 'balls' as is frequently suggested.

You often hear the phrase in the movies "don't break my balls"

....which is a direct translation of the Italian:
"Non rompermi i coglioni" - (coglioni meaning balls/testicles)

A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.

dumb dick idiot (loosely translated from the Germano-European Dialect)