1. (

• firm ,
• steadfast ,
• steady ,
• stiff ,
• unbendable ,
• unfaltering ,
• unshakable ,
• unwavering

) marked by firm determination or resolution; not shakable; "firm convictions"; "a firm mouth"; "steadfast resolve"; "a man of unbendable perseverence"; "unwavering loyalty" ;

2. (

• unassailable ,
• unshakable ,
• watertight ,
• bulletproof

) without flaws or loopholes; "an ironclad contract"; "a watertight alibi"; "a bulletproof argument" ;