Inflexible, inexorable, obdurate, adamant, adamantine mean not to be moved from or changed in a predetermined course or purpose.
All are applicable to persons, decisions, laws, and principles; otherwise, they vary in their applications.
Inflexible usually implies firmly established principles rigidly adhered to; sometimes it connotes resolute steadfastness, sometimes slavish conformity, sometimes mere pigheadedness <society’s attitude toward drink and dishonesty was still inflexible —Wharton > <a morality that is rigid and inflexible and dead —Ellis > <arbitrary and inflexible rulings of bureaucracy —Shils >
Inexorable, when applied to persons, stresses deafness to entreaty <more fierce and more inexorable far than empty tigers or the roaring sea —Shak. > <our guide was inexorable, saying he never spared the life of a rattlesnake, and killed him —Mark Van Doren > When applied to decisions, rules, laws, and their enforcement, it often connotes relentlessness, ruthlessness, and finality beyond question <nature inexorably ordains that the human race shall perish of famine if it stops working —Shaw > It is also often applied to what exists or happens of necessity or cannot be avoided or evaded <inexorable limitations of human nature> <inexorable destiny> <you and I must see the cold inexorable necessity of saying to these inhuman, unrestrained seekers of world conquest … “You shall go no further” —Roosevelt >
Obdurate is applicable chiefly to persons and almost invariably implies hardness of heart or insensitiveness to such external influences as divine grace or to appeals for mercy, forgiveness, or assistance <if when you make your prayers, God should be so obdurate as yourselves, how would it fare with your departed souls? —Shak. > <the obdurate philistine materialism of bourgeois society —Connolly >
Adamant and adamantine usually imply extraordinary strength of will or impenetrability to temptation or entreaty <Cromwell’s adamantine courage was shown on many a field of battle —Goldwin Smith > <when Eve upon the first of men the apple pressed with specious cant, O, what a thousand pities then that Adam was not Adam-ant —Thomas Moore >