| adjective | not to be placated or appeased or moved by entreaty “grim determination” |
| adjective | harsh “the brutal summer sun” |
| adjective | never-ceasing “the relentless beat of the drums” |
| grimness | noun | something hard to endure |
| inexorableness | noun | mercilessness characterized by an unwillingness to relent or let up |
| inexorability | noun | mercilessness characterized by an unwillingness to relent or let up |
| relentlessness | noun | mercilessness characterized by an unwillingness to relent or let up |
| sternness | noun | uncompromising resolution |
| persistency | noun | persistent determination |
| persistence | noun | the property of a continuous and connected period of time |
| persist | verb | be persistent, refuse to stop |