The Collaborative International Dictionary of English v.0.48:

Secretly \Se"cret*ly\, adv.
   In a secret manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

secretly
    adv 1: in secrecy; not openly; "met secretly to discuss the
           invasion plans"; "the children secretly went to the
           movies when they were supposed to be at the library";
           "they arranged to meet in secret" [syn: secretly, in
           secret, on the Q.T., on the QT]
    2: not openly; inwardly; "they were secretly delighted at his
       embarrassment"; "hoped secretly she would change her mind"

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "secretly":
   behind closed doors, behind the curtain, behind the scenes,
   behind the veil, by stealth, confidentially, covertly, furtively,
   hugger-mugger, in a corner, in a whisper, in camera, in darkness,
   in hidlings, in secret, in the background, in the dark,
   mysteriously, nobody the wiser, on the quiet, on the sly,
   privately, privily, quietly, slyly, sotto voce, stealthily,
   sub rosa, surreptitiously, under the breath, under the rose,
   undercover, underground, with bated breath