To lie low. To stay concealed by not standing up(verb)
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Use "keep down" in a sentence
"He tried to keep down his fear, but like rez it kept uncurling, testing the edges of his control."
"At Kupang no means of refitting the worn-out pump or of pitching the leaky seams in the upper works of the boat were obtainable; and Flinders had to face a run across the Indian Ocean with the prospect of having to keep down the water with an impaired equipment."
"How to diminish friction; how to promote ease of intercourse; how to make every part of a man's life contribute to the welfare and satisfaction of those around him; how to keep down offensive pride; how to banish the rasping of selfishness from the intercourse of men; how to move among men inspired by various and conflictive motives, and yet not have collisions -- this is the function of good manners."