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1. To moisten something. A noun or pronoun can be used between "damp" and "down." I have to damp down my hair before combing it, or else it just sticks straight up.
2. To make a fire or flame less intense. A noun or pronoun can be used between "damp" and "down." The room's getting too hot now, so damp down the fire.
To die after overexposure to water. Typically used in reference to plants and seeds. I'm worried that days of torrential rain will cause the flowers I just planted to damp off.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
[for seedlings] to die from too much water. All the new plants damped off, and we had to buy some from the nursery. The little seedlings damped off and withered away.
damp something down
1. Lit. to make something damp. Damp the clothes down before you iron them, Please damp down the clothes first.
2. Fig. to reduce the intensity of a flame, usually by cutting down on the air supply, as with a damper. Please damp the woodstove down. Damp down the air supply or you are going to end up with a raging inferno.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.