| sticky |
| stiff(adjective, harsh, cruel, determined, fierce) |
| stiff-necked(adjective, set, firm, persistent, sure) |
| stop at nothing(expression, adjective, idiom, harsh, energetic) |
| strict(adjective, harsh, cruel, determined, fierce) |
| stringent(adjective, harsh, cruel, determined, fierce) |
| tireless |
| unabated(adjective, persistent, constant, pitiless, unyielding) |
| unappeasable(adjective, harsh, cruel, determined, fierce) |
| unbending(adjective, harsh, cruel, determined, fierce) |
| unbroken(adjective, persistent, constant, pitiless, unyielding) |
| uncompromising(adjective, harsh, cruel, determined, fierce) |
| undeviating(adjective, harsh, cruel, determined, fierce) |
| unfaltering(adjective, steadfast) |
| unflagging |
| unflinching(adjective, harsh, cruel, determined, fierce) |
| unforgiving(adjective, harsh, cruel, determined, fierce) |
| unremitting(adjective, energetic, determined, vigorous, persistent) |
| unshakable |
| unstoppable(adjective, harsh, cruel, determined, fierce) |