tongue hangs out, one's
One is eagerly anticipating something, as in Their tongues were hanging out at the thought of seeing the movie stars in person. This expression alludes to an animal's tongue hanging out of its mouth in anticipation of food. [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.