In American English, "Goombah" is a slang term for an Italian American male, now generally viewed as a slur when used by a person not of Italian descent, but sometimes also applied by Italian Americans to themselves in an ironic or affectionate sense.
Most of the early Italian immigrants to the United States came from the poorest regions of southern Italy, and thus the term "Goombah" apparently derives from a word in southern Italian dialects meaning "friend," "comrade," or "companion." The most likely candidate is Neapolitan cumpà, the apocoped, oxytone slang form of the word cumpari found in Southern Italian dialects (compare with compare in standard Italian and compadre in Spanish). In their original senses, these words meant "godfather" before over time evolving to mean "comrade."
In its original sense of a friend or simply an Italian American, the term "Goombah" was already widely understood by Americans by the 1950s. The more derogatory sense as a slur seems to have come later, dating to around the 1970s with the popularity of The Godfather films, which contain dialogue such as "I don't care how many guinea mafia goombahs come out of the woodwork." Today, calling somebody a "goombah" (in a non-ironic sense) implies that they are an overly-stereotypical Italian male and/or implies that they are somehow connected to the mafia or are a mafioso-like thug.