Webster's English Dictionary

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impregnable

1. im.preg.na.ble \im-'preg-n*-b*l\ \-ble-\ aj [ME imprenable, fr. MF, fr. 
   in- + prenable vulnerable to capt]ure, fr. prendre to take - more at PRIZE 
   : incapable of being taken by assault : UNCONQUERABLE; also  : UNASSAILABLE 
   - im.preg.na.ble.ness n
2. impregnable aj : capable of being impregnated