unfoolable

English

Etymology

From un- +‎ foolable.

Adjective

unfoolable (not comparable)

1. Impossible to fool, not foolable.
   • 1934, Henry G[eorge] Lamond, An Aviary on the Plains, Sydney, N.S.W.: Angus & Robertson Limited, →OCLC, page 206:

     Then what can be the reason, without it is just a grim jest being played on the most unfoolable bird in the bush?