The popularity of this word stems from The Sopranos, an HBO TV show about the New Jersey mafia. Goo-mah is another way to say "mistress". Since this has been spelt phonetically, you should also expect the word to be spelt "goo-ma". Even though The Sopranos is an show about the Italian New Jersey mob, the word Goo-mah is of Chinese origin, where in the Cantonese dialect it means "auntie". In the Soprano's the phrase refers to any one of a mobster's mistresses.
Where have you been? But of course, you've been humping your "goo-mah" all night! The wife's bad or just boring in bed - that's why we all need a goo-mah to get our rocks off!