The Collaborative International Dictionary of English v.0.48:

Impersuasible \Im`per*sua"si*ble\, a. [Pref. im- not +
   persuasible: cf. OF. impersuasible.]
   Not persuasible; not to be moved by persuasion; inflexible;
   impersuadable. --Dr. H. More. -- Im`per*sua`si*bil"i*ty, n.
   [1913 Webster]