inexpugnable (adj.)

late 15c., from Old French inexpugnable (14c.) or directly from Latin inexpugnabilis "not to be taken by assault, not to be rooted out, invincible," from in- "not" (see in- (1)) + expugnabilis "assailable," from expugnare (see expugn). Figurative sense, in reference to arguments, etc., is from 1530s.