| smooth(adjective, tranquil, serene, undisturbed, uninterrupted) |
| solo(adjective, private, single, solitary, alone) |
| specific(adjective, separate, simple, special, individual) |
| strange(adjective, separate, simple, obscure, special) |
| surreptitious(adjective, secret, unknown, hidden, underground) |
| tranquil(adjective, shy, stable, peaceful, serene) |
| unalloyed(adjective, separate, simple, special, individual) |
| unapproachable(adjective, close, quiet, obscure, indifferent) |
| unblended(adjective, separate, simple, special, individual) |
| uncommon(adjective, separate, simple, special, individual) |
| uncompounded(adjective, separate, simple, special, individual) |
| under an eclipse(expression, idiom, concealment) |
| under cover(expression, obscure, dark, isolated, reclusive) |
| under wraps(expression, clandestine, underground, undisclosed) |
| undisturbed(adjective, shy, stable, peaceful, tranquil) |
| undivided(adjective, separate, simple, special, individual) |
| unitary(adjective, separate, simple, special, individual) |
| unknown(obscure, dark, secret, hidden, concealed) |
| unmixed(adjective, separate, simple, special, individual) |
| unrivaled(adjective, separate, simple, special, individual) |