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1. To inflate something with a gas (or, less commonly, a liquid) using a pump. A noun or pronoun can be used between "pump" and "up." Often used in passive constructions. Would you please pump up the air mattress for when my sister comes to stay with us? They pumped the balloons up with water. Most chicken breasts sold at the supermarket are pumped up with water before being frozen.
2. To increase someone's or one's own excitement, confidence, or mental preparation. A noun or pronoun can be used between "pump" and "up." The coach pumped us all up just before we headed out onto the field. I was pretty nervous before the test, so I took a few minutes to pump myself up before I started.
3. To increase the volume of something. A noun or pronoun can be used between "pump" and "up." Oh, I love this song—pump it up! Pump up the volume so everyone can hear the music!
4. To exaggerate or falsely inflate the numbers or information about something. A noun or pronoun can be used between "pump" and "up." The company has been accused of manipulating cash in its accounts to pump up its profits when reporting to shareholders. I just think the government is pumping these figures up to justify the new legislation.
mod. excited; physically and mentally ready. (Sports.) The team is really pumped up for Friday’s game.