Plenipotentiary \plen-uh-puh-TEN-shee-air-ee\ or \plen-uh-puh-TEN-shuh-ree\ , adjective;
1. Containing or conferring full power; invested with full power; as, "plenipotentiary license; plenipotentiary ministers"
1. A person invested with full power to transact any business; especially, an ambassador or diplomatic agent with full power to negotiate a treaty or to transact other business
This word dates from the 1640's and comes from Middle Latin plenipotentiarius ("having full power") which derives from Latin plenus + potentum ("full" + "powerful"). Potentum is the present participle of *potere ("be powerful") which comes from potis ("powerful, able, capable"). Plenus derives from Proto-Indo-European *ple- ("to be full").
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Saturday, January 1
Etymologies come from the Oxford English Dictionary and/or Etymonline.com