A.Word.A.Day with Anu Garg

1. Incapable of being taken by force; strong enough to withstand attack.
2. Capable of being impregnated.
For 1: From Old French imprenable, from in- (not) + prenable (vulnerable to capture), from prendre (to take, seize). Ultimately from the Indo-European root ghend-/ghed- (to seize, take), which is also the source of pry, prey, spree, reprise, surprise, pregnant, osprey, prison, and get. Earliest documented use: 1430.
For 2: From Latin impraegnaere (to fertilize, impregnate). Ultimately from the Indo-European root gen- (to give birth), which also gave birth to words such as generate, engine, indigenous, and germ.

impegnable - to be capable of being pegged or not.