1. [adj]  having austere inflexibility; "a flinty manner"; "granitic morality"; "his unyielding mouth and glassy eyes"- Marchette Chute
2. [adj]  refusing to give way or compromise; "unyielding determination"
3. [adj]  stubbornly unyielding; "dogged persistence"; "dour determination"; "the most vocal and pertinacious of all the critics"; "a mind not gifted to discover truth but tenacious to hold it"- T.S.Eliot; "men tenacious of opinion"

UNYIELDING is a 10 letter word that starts with U.