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To fasten or secure something in place with buttons. A noun or pronoun can be used between "button" and "down." Button down your pocket so that nothing falls out of it. Button that flap down so it doesn't get blown around by the wind.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
button something down
to fasten something down with buttons. Button your collar down. You look too casually dressed. Please button down your collar.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.