| sneaky(clandestine, underground, undisclosed) |
| soft-pedalled(quiet, plain, imperceptible, discreet, inconspicuous) |
| sub-rosa(close, clandestine, underground, undisclosed, furtive) |
| submerged |
| surreptitious(secret, hidden, clandestine, underground, undisclosed) |
| tenuous(quiet, plain, imperceptible, discreet, inconspicuous) |
| top secret(expression, close, clandestine, underground, undisclosed) |
| ulterior(secret, hidden, underground, underhanded, covered) |
| unacknowledged(close, clandestine, underground, undisclosed, furtive) |
| unbeheld |
| unbeholdable |
| 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) |
| undercover(obscure, secret, hidden, clandestine, indirect) |
| underhand(secret, hidden, clandestine, underground, undisclosed) |
| underhanded(secret, hidden, clandestine, underground, undisclosed) |
| undiscernible |
| unemphatic(quiet, plain, imperceptible, discreet, inconspicuous) |