clandestine

Definitions:

Hidden; secret; concealed; underhand; implying evil intent.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)
Secret; hidden; private- applied to wrong actions.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
Secret; concealed.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (adjective) (adj.) (adjective)
Concealed or hidden: private: unlawful: sly.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (adjective) (adj.) (adjective)
Clamp secret; private.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
Clandestinely.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adverb) (adv.) (adverb)

Usage examples:

In fact it would be nearly impossible for any one to gain a forcible or clandestine admittance to any part of the establishment.
- Life in the Grey Nunnery at Montreal by Sarah J Richardson
His manner was so exaggerated in its significance that Miss Popkiss, with Mr. Sparrow's clandestine visit fresh in her mind, grew uneasy under it.
- A Poached Peerage by William Magnay
This reduced poor Mrs. Little to silence; followed, in due course, by a few meek, clandestine tears.
- Put Yourself in His Place by Charles Reade