The Collaborative International Dictionary of English v.0.48:

Japery \Jap"er*y\, n. [Cf. OF. japerie a yelping.]
   Jesting; buffoonery. [Obs.] --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

japery
    n 1: acting like a clown or buffoon [syn: buffoonery,
         clowning, japery, frivolity, harlequinade, prank]