1. (

• impregnable ,
• inviolable ,
• secure ,
• strong ,
• unassailable ,
• unattackable

) immune to attack; incapable of being tampered with; "an impregnable fortress"; "fortifications that made the frontier inviolable"; "a secure telephone connection" ;

2. (

• unassailable ,
• untouchable

) impossible to assail ;

3. (

• unassailable ,
• unshakable ,
• watertight ,
• bulletproof

) without flaws or loopholes; "an ironclad contract"; "a watertight alibi"; "a bulletproof argument" ;