WATERTIGHT
unassailable, unshakable, watertight, bulletproof
(adjective) without flaws or loopholes; “an ironclad contract”; “a watertight alibi”; “a bulletproof argument”
watertight
(adjective) not allowing water to pass in or out
Source: WordNet® 3.1
Etymology
Adjective
watertight (comparative more watertight, superlative most watertight)
So tightly made that water cannot enter or escape.
(figurative) So devised or planned as to be impossible to defeat, evade or nullify.
Synonyms
• waterproof, water-proof, impervious, impermeable
Source: Wiktionary
Wa"ter-tight`, a.
Definition: So tight as to retain, or not to admit, water; not leaky.
Source: Webster’s Unabridged Dictionary 1913 Edition