| inexorable(adjective, harsh, cruel, determined, fierce) |
| inhuman(adjective, harsh, cruel, determined, fierce) |
| intense(adjective, firm, solid, deliberate, determined) |
| intractable(adjective, firm, determined, fixed, resolute) |
| intransigent(adjective, determined, steady, fixed, impassive) |
| intrepid |
| inveterate(adjective, firm, determined, usual, regular) |
| iron-fisted(adjective, severe, stiff, tight, strict) |
| mean business(expression, adjective, cruel, determined, pitiless) |
| meaning business |
| merciless(adjective, harsh, cruel, determined, fierce) |
| mortal(adjective, harsh, cruel, determined, fierce) |
| mulish(adjective, firm, determined, fixed, resolute) |
| never-failing(adjective, strong, firm, solid, certain) |
| nose to grindstone |
| opinionated(adjective, determined, fixed, stubborn, hardheaded) |
| painstaking |
| pat(adjective, firm, solid, determined, usual) |
| patient |
| persevering(adjective, strong, firm, energetic, certain) |