黄鱼 (huáng yú) literally refers to the yellow croaker, a common fish in Chinese cuisine. In informal transportation contexts, it also means an extra passenger taken illegally by a driver who pockets the fare instead of reporting it — a term originating from the idea of drivers hiding extra passengers like yellow croakers in a fish tank. You'll often hear it in compounds like 黄鱼车 (an unlicensed ride with extra passengers) and 拉黄鱼 (to pick up such passengers).