The Collaborative International Dictionary of English v.0.48:

Jeering \Jeer"ing\, a.
   Mocking; scoffing. -- n. A mocking utterance. --
   Jeer"ing*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

jeeringly
    adv 1: in a disrespectful jeering manner [syn: jeeringly,
           mockingly, gibingly]