Literally 'pretend to be garlic' (装 pretend + 蒜 garlic). The origin is debated — one theory says garlic sprouts (蒜苗) look like leek sprouts (韭菜苗) when young, so 'pretending to be garlic' means pretending to be something you're not.
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Very colloquial and mildly confrontational. Used when calling someone out for feigning ignorance. Similar to English 'don't play dumb with me.'
