hen's tooth

Something very scarce (because hens have no teeth). Typically used in phrases like "as scarce as a hen's tooth." I can hardly believe your brother got an A on his final exam—grades like that are as scarce as a hen's tooth for him! Please, who would I even ask to the prom? A high school boy who isn't a doofus is as rare as a hen's tooth. Support for the president is as scarce as a hen's tooth in this part of the country.