The Collaborative International Dictionary of English v.0.48:

Detestably \De*test"a*bly\, adv.
   In a detestable manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

detestably
    adv 1: in an offensive and hateful manner; "I don't know anyone
           who could have behaved so abominably" [syn: detestably,
           repulsively, abominably, odiously]