Open English Wordnet
Adjectives
(s)firm, steadfast, steady, stiff, unbendable, unfaltering, unshakable, unwaveringmarked by firm determination or resolution; not shakable“firm convictions”, “a firm mouth”, “steadfast resolve”, “a man of unbendable perseverance”, “unwavering loyalty”
(s)level, unwaveringnot showing abrupt variations“spoke in a level voice”, “she gave him a level look” (Louis Auchincloss)