The Collaborative International Dictionary of English v.0.48:

Ferly \Fer"ly\, a. [AS. f?rlic sudden, unexpected. See Fear,
   n.]
   Singular; wonderful; extraordinary. [Obs.] -- n. A wonder; a
   marvel. [Obs.]
   [1913 Webster]

         Who hearkened ever such a ferly thing.   --Chaucer.
   [1913 Webster] Ferm