ultrarare

From ultra- + rare.

1. 01
   derived from *h₁reh₁-✻ — “friable, thin”
2. 02
   derived from rārus — “loose, spaced apart, thin, infrequent”
3. 03
   derived from rare
4. 04
   inherited from rare
5. 05
   prefixed as ultrarare — “ultra + rare”