know one's stuff 精通自己的业务，在行，内行 ；精明强干 e.g. Once you know your stuff, you’ll look like an expert, too.
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?有两下子
...e of people with lofty ideals, moral integrity, fine education and a high sense of discipline 有两下子know one's stuff / One is really something.? 有朋自远方来，不亦乐乎 How happy we are to meet friends from afar.