prick up one's ears - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Listen carefully, pay close attention, as in When she heard them mention her boyfriend she pricked up her ears. This term alludes to horses raising their ears at a sudden noise. [Late 1500s]