(as) rare as hens' teeth

Incredibly scarce or rare; extremely difficult or impossible to find. Support for the president is as rare as hens' teeth in this part of the country.

rare as hen's teeth

scarce as hen's teeth

OLD-FASHIONED

If something is as rare as hen's teeth or as scarce as hen's teeth, it is extremely rare. Record companies are becoming as rare as hen's teeth. Note: Hens do not have teeth.

rare (or scarce) as hen's teeth

extremely rare.

As hens do not possess teeth, the implication is that something is rare to the point of non-existence. The phrase was originally a US colloquialism, dating from the mid 19th century.

(as) rare/scarce as hen’s ˈteeth

(old-fashioned) extremely rare: Critics always complain that good movies that the whole family can see together are as scarce as hen’s teeth.

This refers to the fact that hens do not have teeth.