A word used by non-Cantonese (a dialect of the Chinese language) speaking people. It orginated as a term thought to be Cantonese, but has been proven thus far not to be a part of the Cantonese lexicon.
The word is most commonly used as an informal greeting, but there are known instances of it being used formally. It is also used often used as a noun; in which it is almost certainly used in a derogatory manner.
Yo, that gumagum thought I was a bouncer and showed me his ID!