The Collaborative International Dictionary of English v.0.48:

discreditable \dis*cred"it*a*ble\, a.
   Not creditable; injurious to reputation; disgraceful;
   disreputable. -- Dis*cred"it*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

discreditably
    adv 1: in a dishonorable manner or to a dishonorable degree;
           "his grades were disgracefully low" [syn:
           disgracefully, ingloriously, ignominiously,
           discreditably, shamefully, dishonorably,
           dishonourably]