1) Behind one`s back 2) Furtively 3) In camera 4) Incognito 5) On the qt 6) Privily 7) Secretively 8) Stownlins 9) Subrosa 1) Clandestinely 2) Covertly 3) How plots are planned 4) In a clandestine manner 5) In secrecy 6) In secret 7) Inwardly 8) Not openly 9) On the qt 10) On the sly 11) Privately 12) Silently 13) Stealthily 14) Sub rosa 15) To yourself 
• (adv.) In a secret manner.
Se'cret·ly adverb In a secret manner. 
[adv] - in secrecy 2. [adv] - not openly
secretly 1. Characterized by something that is kept hidden from others or is known only to oneself or to a few. 2. Relating to something that remains beyond understanding or explanation; a mystery. 3. A reference to a method or formula on which success is based.
in secret adverb 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`