UNSHAKABLE

unassailable, unshakable, watertight, bulletproof

(adjective) without flaws or loopholes; “an ironclad contract”; “a watertight alibi”; “a bulletproof argument”

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

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

Source: WordNet® 3.1

Etymology

Adjective

unshakable (comparative more unshakable, superlative most unshakable)

Not able to be shaken; firm, solid

Antonym: shakable

resolute, unfaltering, unwavering

(figuratively) Having no errors or loopholes; unassailable

Source: Wiktionary

Un*shak"a*ble, a.

Definition: Not capable of being shaken; firm; fixed. Shak. J. S. Mill.

Source: Webster’s Unabridged Dictionary 1913 Edition