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. (

• conceptive ,
• impregnable

) capable of conceiving ;

3. (

• impregnable ,
• inexpugnable

) incapable of being overcome, challenged or refuted; "an impregnable argument"; "impregnable self-confidence" ;