puissant \PWISS-uhnt; PYOO-uh-suhnt; pyoo-ISS-uhnt\ (adjective) - Powerful; strong; mighty; as, a puissant prince or empire.
"Awake remembrance of these valiant dead,
And with your pussiant arm renew their feats.
You are their heir, you sit upon their throne.
The blood and courage that renowned them
Runs in your veins, and my thrice-pussiant liege." -- William Shakespeare, 'Henry V'
Puissant is from Old French puissant, "powerful," ultimately from (assumed) Vulgar Latin potere, alteration of Latin posse, "to be able." The noun form is puissance.