unpen means to release (from a pen). Lexicurio rates it Sui generis — a strength score of 87 out of 100.
verb
- To release (from a pen).“Her supplies, a billy of tea and scraps of salt beef and damper (her dog got the beef), gave out the first day, though that was as nothing to her compared with the bleat of the penned sheep, for it was summer and droughty, and her dog could not unpen them.”