a.
1.
Obstinate, unyielding, inflexible, positive, contumacious, refractory, perverse, headstrong, wilful, dogged, mulish, unruly, unmanageable, ungovernable, intractable, indocile, heady, cross-grained, obdurate.
2.
Persevering, persistent, steady, constant, unremitting.
3.
Stiff, unpliant, inflexible, firm, tough, hard, strong.
4.
Hardy, firm, enduring, stoical, uncomplaining.
5.
Refractory, difficult of fusion.