hide one's light under a bushel
hide one's light under a bushel {v. phr.}
To be very shy and modest and not show your abilities or talents; be too modest in letting others see what you can do.
When Joan is with her close friends she has a wonderful sense of humor, but usually she hides her light under a bushel.
Mr. Smith is an expert in many fields, but most people think he is not very smart because he hides his light under a bushel.
All year long Tommy hid his light under a bushel and the teacher was surprised to see how much he knew when she read his exam paper.
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Source: A Dictionary of American Idioms