Synonyms of the word

watertight

• adj. So tightly made that water cannot enter or escape.
• adj. So devised or planned as to be impossible to defeat, evade or nullify.

bulletproof

• adj. (of a material) Capable of withstanding a direct shot by a bullet fired from a gun.
• adj. (idiomatic) reliable, infallible, sturdy or error-tolerant.
• adj. (usually of an idea or concept) Unbreakable, very tough.
• v. To make proof against bullets.
• v. (slang) to make resistant to failure.

incontestable

• adj. Not contestable; indisputable; certain; incontrovertible.

incontestible

• adj. Alternative spelling of incontestable.

tight

• adj. Firmly held together; compact; not loose or open.
• adj. (of a space, design or arrangement) Narrow, such that it is difficult for something or someone to pass…
• adj. Well-rehearsed and accurate in execution.
• adj. (slang) Intoxicated; drunk or acting like being drunk.
• adj. (slang) Extraordinarily great or special.
• adj. (slang, British (regional)) Mean; unfair; unkind.
• adj. (obsolete) Not ragged; whole; neat; tidy.
• adj. (obsolete) Handy; adroit; brisk.
• adj. (poker) Of a player, who plays very few hands.
• adj. (poker) Using a strategy which involves playing very few hands.
• adv. Firmly, so as not to come loose easily.
• adv. Soundly.
• v. (obsolete) To tighten.

unassailable

• adj. secure against attack; impregnable.
• adj. (by extension) undeniable, incontestable or incontrovertible.

unshakable

• adj. Not able to be shaken; firm, solid.
• adj. resolute, unfaltering, unwavering.
• adj. (figuratively) Having no errors or loopholes; unassailable.