| furtive(adjective, close, mysterious, reticent, taciturn) |
| clandestine(adjective, underground, undisclosed) |
| covert(adjective, mysterious, private, reticent, taciturn) |
| surreptitious(adjective, clandestine, underground, undisclosed) |
| undercover(adjective, mysterious, private, reticent, taciturn) |
| secret(adjective, sensitive, clandestine, underground, undisclosed) |
| stealthy(adjective, clandestine, underground, undisclosed) |
| underground(adjective, clandestine, undisclosed) |
| underhand(adjective, clandestine, underground, undisclosed) |
| backstair |
| sneaky(adjective, clandestine, underground, undisclosed) |
| underhanded(adjective, clandestine, underground, undisclosed) |
| hole-and-corner |
| hugger-mugger |
| secretive(adjective, mysterious, private, restricted, reticent) |
| sly(adjective, clandestine, underground, undisclosed) |
| backdoor(close, sensitive, inside, clandestine, underground) |
| hidden(unknowing, clandestine, underground, undisclosed, hermetical) |
| skulking |
| sneaking |