| strange(adjective, new, remote, indefinite, clandestine) |
| sub-rosa(close, secret, hidden, spy, clandestine) |
| such-and-such(strange, new, remote, indefinite, unidentified) |
| surreptitious(secret, hidden, spy, clandestine, underground) |
| top secret(expression, close, clandestine, underground, undisclosed) |
| unapprehended(strange, new, remote, indefinite, unidentified) |
| unascertained(strange, new, remote, indefinite, unidentified) |
| uncelebrated(strange, new, remote, unknown, indefinite) |
| uncharted(strange, new, remote, indefinite, unidentified) |
| undefined |
| under-the-table(secret, hidden, clandestine, underground, undisclosed) |
| under false pretense(expression, idiom, close, clandestine, underground) |
| under the radar |
| under wraps(expression, secret, hidden, clandestine, underhanded) |
| underhand(secret, hidden, spy, clandestine, underground) |
| underhanded(secret, hidden, clandestine, underground, undisclosed) |
| undesignated |
| undisclosed(unknown, clandestine, underground, nameless) |
| undiscovered(strange, new, remote, indefinite, unidentified) |
| undistinguished(strange, new, remote, unknown, indefinite) |