French form of William, from Germanic elements meaning 'resolute protector.'
Guillaume is the French form of William, one of the most consequential names in Western history, derived from the Old High German Willahelm — a compound of willo, meaning 'will' or 'desire,' and helm, meaning 'helmet' or 'protection.' The name thus carries the martial sense of 'resolute protector' or 'determined guardian.' It arrived in England with a vengeance in 1066: Guillaume le Conquérant, William the Conqueror, Duke of Normandy, defeated Harold at Hastings and fundamentally transformed English language, culture, and governance, making William the dominant name in England for centuries.
In France, the name's cultural legacy is equally immense. Guillaume de Machaut (circa 1300–1377) was the greatest composer and poet of the medieval Ars nova, essentially inventing polyphonic song forms that shaped Western music for centuries. Guillaume de Lorris wrote the Roman de la Rose, the most widely read secular poem of the Middle Ages.
And Guillaume Apollinaire (1880–1918) — born Wilhelm Albert Włodzimierz Apollinaris de Wąż-Kostrowicki — took the name as his pen name and became one of the foundational poets of modernism, coining the word 'surrealism' and writing with shattering originality right up to his death from Spanish flu. Today Guillaume is quintessentially French — it functions as a signal of Francophile sensibility when used outside France, carrying the elegance of the language in its very syllables. Its pronunciation (roughly 'gee-YOME') is a small test of cultural fluency, and those who pass it are rewarded with a name of extraordinary historical depth.