1. incapable of change

- a man of inflexible purpose

Similar word(s): adamant, adamantine, inexorable, intransigent, fossilised, fossilized, ossified, brassbound, ironclad

2. not making concessions

Similar word(s): hardline, sturdy, uncompromising

3. resistant to being bent

- an inflexible iron bar

- an inflexible knife blade

Similar word(s): inelastic, rigid, stiff, semirigid

4. incapable of adapting or changing to meet circumstances

- an inflexible law

Similar word(s): unadaptable, rigid, unbending