weapons plutonium wea·ponsp·lu·tonium uncommon noun 1. plutonium 239 that is recovered when nuclear weapons are disassembled; it is stored in plutonium pits Synonymsweapons plutoniumweapon-grade plutonium Related words Broader terms plutonium 239 WordNet 2.1 (Princeton University). WordNet 2.1 Copyright Princeton University. All rights reserved.