Also, scarcer than hen's teeth. Exceptionally rare, as in On a rainy night, taxis are as scarce as hen's teeth. Since hens have no teeth, this term in effect says that something is so scarce as to be nonexistent. [Mid-1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.