1998 July 15, Alex Milman, “Welsh archers”, in soc.history.medieval[1] (Usenet), retrieved 2022-06-07:
A simple forest was an unpregnable obstacle (Agincourt had been beaten to death).
2000 August 18, PMohan, “How to cage the dogs from across the borders?”, in soc.culture.indian[2] (Usenet), retrieved 2022-06-07:
GOI has been able to withdraw military from Kashmir and put them on border (LOC) only (Fence). This has cut our costs and made our border more unpregnable.
2002 June 21, Paul, “What to think of FICHET french locks”, in alt.locksmithing[3] (Usenet), retrieved 2022-06-07:
What do you think of FRENCH "FICHET 787" locks, with a very special key, that are called "unpickable" and "unpregnable" by Fichet ?