If it comes from Hong Kong movies, I personally think it comes from "eggplant jelly coffee" (walk-on/extras/supporting roles and other unimportant little people). It is said that this word also comes from the pronunciation of "carefree", "carefree" and "relaxed and happy", which refers to this kind of people (extended to unimportant people).
For details, please refer to Baidu's "Eggplant Jelly Coffee" and "Strong Coffee".