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1. To push or press something downward with great force or speed. The fighter pilot punched the throttle down, and the jet rocketed forward. I punched down the drill into the brick.
2. To press down firmly in the center of dough with one's fist after it has risen in order to eliminate gas bubbles and redistribute yeast. A noun or pronoun can be used between "punch" and "down." Make sure you punch down your dough before you bake it, or your bread will be full of huge air bubbles when it comes out of the oven. Your dough will end up much tastier if you punch it down after it finishes rising.
3. To break up and submerge the solid mass of grape skins, seeds, and stems (known as the "cap") that rises to the top of wine during the fermentation process. A noun or pronoun can be used between "punch" and "down." A: "I tried making red wine at home, but it tasted pretty weak." B: "You probably forgot to punch down the cap." Punching the cap down will give your wine better color and astringency, not to mention flavor.
4. To insert and secure a copper telecommunication wire into some kind of open-ended connector. A noun or pronoun can be used between "punch" and "down." The phrase can be used in reference to the wire or the connector. You've got to make sure the wire is nice and straight when you go to punch it down. It's clear just from looking at this that whoever installed this thing had no idea how to punch down a patch panel.
5. slang To make jokes at the expense of a person or group that is in a position of social, political, or economic weakness relative to oneself. The comedian suddenly started telling jokes about people in the ghettos, and I got really uncomfortable. Comedians shouldn't punch down, if you ask me.
punch something down
to press something down. Punch this lever down and then try to place your telephone call. Punch down this lever and push this button.