Synonyms of the word

impregnable

• adj. (of a fortress, wall, etc., also used figuratively) Too strong to be penetrated.
• adj. Capable of being impregnated.

conceptive

• adj. relating to conception (in all senses).

fertile

• adj. (of land etc) capable of growing abundant crops; productive.
• adj. (biology) capable of reproducing; fecund, fruitful.
• adj. (biology) capable of developing past the egg stage.
• adj. (of an imagination etc) productive or prolific.

inexpugnable

• adj. Impossible to eliminate or destroy; impregnable.

inviolable

• adj. Not violable; not to be infringed.
• adj. Not susceptible to violence, or of being profaned, corrupted, or dishonoured.
• adj. Incapable of being injured or invaded; indestructible.

invulnerable

• adj. Incapable of being injured; not vulnerable.
• adj. Unanswerable; irrefutable.

secure

• adj. Free from attack or danger; protected.
• adj. Free from the danger of theft; safe.
• adj. Free from the risk of eavesdropping, interception or discovery; secret.
• adj. Free from anxiety or doubt; unafraid.
• adj. Firm and not likely to fail; stable.
• adj. Free from the risk of financial loss; reliable.
• adj. Confident in opinion; not entertaining, or not having reason to entertain, doubt; certain; sure; commonly…
• adj. Overconfident; incautious; careless.
• v. To make safe; to relieve from apprehensions of, or exposure to, danger; to guard; to protect.
• v. To put beyond hazard of losing or of not receiving; to make certain; to assure; frequently with against…
• v. To make fast; to close or confine effectually; to render incapable of getting loose or escaping.
• v. To get possession of; to make oneself secure of; to acquire certainly.

strong

• adj. Capable of producing great physical force.
• adj. Capable of withstanding great physical force.
• adj. (of water, wind, etc.) Having a lot of power.
• adj. Determined; unyielding.
• adj. Highly stimulating to the senses.
• adj. Having an offensive or intense odor or flavor.
• adj. Having a high concentration of an essential or active ingredient.
• adj. (specifically) Having a high alcoholic content.
• adj. (grammar) Inflecting in a different manner than the one called weak, such as Germanic verbs which change…
• adj. (chemistry) That completely ionizes into anions and cations in a solution.
• adj. (military) Not easily subdued or taken.
• adj. (slang, US) Impressive, good.
• adj. Having a specified number of people or units.
• adj. (of a disease or symptom) severe (very bad or intense).
• adj. (mathematics, logic) Having a wide range of logical consequences; widely applicable. (Often contrasted…
• adj. (of an argument) Convincing.
• adv. In a strong manner.

unassailable

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

unattackable

• adj. which may not be attacked.

unconquerable

• adj. Not conquerable; indomitable.