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Phrase(s): a jack of all trades is a master of none.
Prov. If you are able to do a lot of things fairly well, you will not have time to learn to do one thing extremely well. • Jill: I envy Bob; he can do so many things. He writes novels, paints pictures, makes sculptures, and even plays the dulcimer. Jane: It’s true he does a lot of things, but he probably doesn’t do them all terribly well. A jack of all trades is a master of none, you know.
McGrawhill's American Idioms And Phrasal Verbs