装蒜 is an informal, mildly pejorative verb meaning 'to pretend not to know, to act innocent, or to play dumb'. It's a separable verb-object compound (装 + 蒜), so measure words or modifiers can split the two characters — for example 装什么蒜 ('what are you pretending for?') or 装了一回蒜 ('pretended once'). You use it when someone is clearly aware but feigns ignorance, often to avoid a question or responsibility.