Synonyms of the word

unshakable

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

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.

firm

• n. (Britain, business) A business partnership; the name under which it trades.
• n. (business, economics) A business enterprise, however organized.
• n. (slang) A criminal gang, especially based around football hooliganism.
• adj. steadfast, secure, hard (in position).
• adj. fixed (in opinion).
• adj. solid, rigid (material state).
• v. (transitive) To make firm or strong; fix securely.
• v. (transitive) To make compact or resistant to pressure; solidify.
• v. (intransitive) To become firm; stabilise.
• v. (intransitive) To improve after decline.
• v. (intransitive, Australia) To shorten (of betting odds).
• v. (transitive, Britain, slang) To select (a higher education institution) as one's preferred choice, so…

incontestable

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

incontestible

• adj. Alternative spelling of incontestable.

resolute

• adj. Firm, unyielding, determined.
• adj. (obsolete) Convinced; satisfied; sure.

steadfast

• adj. Fixed or unchanging; steady.
• adj. Firmly loyal or constant; unswerving.

steady

• adj. Firm in standing or position; not tottering or shaking; fixed; firm.
• adj. Constant in feeling, purpose, or pursuit; not fickle, changeable, or wavering; not easily moved or persuaded…
• adj. Smooth and not bumpy or with obstructions.
• adj. Regular and even.
• adj. Slow.
• v. To stabilize something; to prevent from shaking.
• n. (informal) A regular customer.

stiff

• adj. (of an object) Rigid, hard to bend, inflexible.
• adj. (figuratively, of policies and rules and their application and enforcement) Inflexible; rigid.
• adj. (of a person) Formal in behavior; unrelaxed.
• adj. (colloquial) Harsh, severe.
• adj. (of muscles or parts of the body) Painful as a result of excessive or unaccustomed exercise.
• adj. Potent.
• adj. Dead, deceased.
• adj. (of a penis) Erect.
• adj. (cooking, of whipping cream or egg whites) Beaten until so aerated that they stand up straight on their…
• adj. (mathematics) Of an equation: for which certain numerical solving methods are numerically unstable, unless…
• n. An average person, usually male, of no particular distinction, skill, or education, often a working stiff…
• n. A person who is deceived, as a mark or pigeon in a swindle.
• n. (slang) A cadaver, a dead person.
• n. (US) A person who leaves (especially a restaurant) without paying the bill.
• n. (blackjack) Any hard hand where it is possible to exceed 21 by drawing an additional card.
• v. To fail to pay that which one owes (implicitly or explicitly) to another, especially by departing hastily.
• v. to cheat someone.
• v. to tip ungenerously.

unassailable

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

unbendable

• adj. not bendable.

unfaltering

• adj. Without faltering, continuous, steadfast.

unwavering

• adj. Never doubted; always steady and on course.

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.