He's ignorant and incompetent - basically a useless freeloader.
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Traceable to Wang Chong's 论衡 (Eastern Han): 腹为饭坑，肠为酒囊 - 'the belly is a rice pit, the bowels a wine sack' - describing those who only eat and drink without thinking.
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Strongly insulting. Use only when describing someone harshly; not appropriate in polite or professional speech.
