#1 unshakable s. [firm, steadfast, steady, stiff, unbendable, unfaltering, unwavering] marked by firm determination or resolution; not shakable; "firm convictions"; "a firm mouth"; "steadfast resolve"; "a man of unbendable perseverence"; "unwavering loyalty". wn #2 unshakable s. [unassailable, watertight, bulletproof] without flaws or loopholes; "an ironclad contract"; "a watertight alibi"; "a bulletproof argument". wn