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)

Whenever making a congratulatory remark, this is what you say. Made popular by Kamala Harris when she and Joe Biden won the presidency.

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