| confidential(adjective, restricted, clandestine, underground, discreet) |
| cryptic(adjective, close, mysterious, private, reticent) |
| feline(mysterious, private, reticent, taciturn, covert) |
| hush-hush(clandestine, underground, undisclosed) |
| occult(clandestine, underground, undisclosed) |
| privy |
| shifty |
| under-the-table(sensitive, unknowing, clandestine, underground, undisclosed) |
| arcane(adjective, clandestine, underground, undisclosed) |
| back-door |
| back door |
| behind-the-scenes |
| buttoned up(expression, adjective, sensitive, mysterious, private) |
| cagey(mysterious, private, reticent, taciturn, covert) |
| camouflaged(close, clandestine, underground, undisclosed, furtive) |
| clammed up(expression, mysterious, private, reticent, taciturn) |
| classified(sensitive, clandestine, underground, undisclosed, sub-rosa) |
| close(mysterious, private, reticent, inside, taciturn) |
| close-mouthed(mysterious, private, reticent, taciturn, uncommunicative) |
| closet(adjective, clandestine, underground, undisclosed) |