Source : Webster's Revised Unabridged Dictionary (1913)

Unshakable \Un*shak"a*ble\, a.
   Not capable of being shaken; firm; fixed. --Shak. J. S. Mill.

Source : WordNet®

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