clandestine

Etymology

From Latin clandestīnus (“secret, concealed”); compare French clandestin.

adj

1. Done or kept in secret, sometimes to conceal an illicit or improper purpose.
   clandestine military operations
   Monimia, who while she yielded to his earnest entreaties had always felt, from the natural rectitude of her understanding, the impropriety of their clandestine correspondence, would, he feared, be more than ever sensible of her indiscretion, when she found that a servant was entrusted with it... 1793, Charlotte Smith, chapter 1, in The Old Manor House, volume 3
   Whether the torments of absence were softened by a clandestine correspondence, let us not inquire. Mr. and Mrs. Morland never did. 1818, Jane Austen, chapter 31, in Northanger Abbey
   Wildeve's clandestine plan with her was to take a little gravel in his hand and hold it to the crevice at the top of the window shutter, which was on the outside, so that it should fall with a gentle rustle, resembling that of a mouse, between shutter and glass. 1878, Thomas Hardy, chapter 11, in The Return of the Native
   Lady Bullingdon could not, of course, countenance such an arrangement for a moment, and the two unhappy persons escaped for a clandestine marriage. 1912, G. K. Chesterton, chapter 9, in Manalive
   Bodies were always a nuisance -- even the small guinea-pig bodies from the slight clandestine experiments in West’s room at the boarding-house. 1922, H. P. Lovecraft, Herbert West: Reanimator
   Probably, the Master-at-arms' clandestine persecution of Billy was started to try the temper of the man... 1924, Herman Melville, chapter 13, in Billy Budd
   As I stand at the edge of existence; nameless shores of infinity. Perpetual unbirth unraveling distance, clandestine truths unveiled. 2003, “Fugue”, in Trivium (music), Ember To Inferno
   In my imagination, all work place encounters between men and women result in clandestine sex. 2004, Penny Arcade
   2005 — Stewart Lee, 90's Comedian DVD And I don't want anyone to think I dislike Catholicism because I don't. It's actually my favourite form of clandestine global evil.
2. (Freemasonry, of a person or lodge) Not recognized as a regular member.