An awkward neighbor or boyfriend/girlfriend in your dorm who invites himself/herself to meals, parties, and other social events. Ass monkeys attach themselves to specific groups or individuals by seeing that other individuals are able to invite themselves to events but still be welcomed companions.
Sketchball A: We need to go out the side door. If we don't your ass monkey will come with us to dinner.
Sketchball B: Tru that 'ho. I don't want my ass monkey fucking up my Pimp Committee.
Mutual Friend of Sketchballs A and B: I swear Sketchball B. If your ass monkey comes to one more meal with us I'm going to jump off a bridge.
Noun. When white people (see stuff white people like) use a single foreign song that has entered mainstream American music, generally through movies, to gain cultural entre into the music of an exotic country/region. By using pandora, related videos feature of youtube, or other tools for finding similar songs the white person is able to build a small, narrow understanding of music from a foreign place. The role of the gateway song is vital in the white person's understanding of a foreign music source because it will be the standard by which all other songs "discovered" in the genre will be judged. However, it is important to realize that the white person will never veer far from the gateway song.
Finally, it must be noted that white people are acutely ashamed of the gateway song because it proves that he/she was completely unaware of a genre of foreign music until exposed by the distinctly unauthentic mainstream media. White people would prefer to be exposed to "world" music from browsing non-chain record stores, study abroad programs, and non-white friends. Bonus would be a combination of a musical "discovery" while falling in love with a foreign person. However, since these more authentic methods of "discovering" "world" music are not always available, gateway songs are a necessary evil for the average white person.
Jai Ho-gateway song to bollywood, anything by Bob Marley-gateway song to reggae, anything by Sean Paul-gateway to reggaeton, 12 girls band version of Clocks etc.