Guillermo is a Spanish name derived from the Germanic name Willahelm, which is composed of the elements "wil" meaning will or desire, and "helm" meaning helmet or protection. It is the Spanish form of the French name Guillaume and the German name Wilhelm.

The name Willahelm dates back to the 8th century and was popular among the Frankish nobility during the Carolingian dynasty. It was borne by several Frankish nobles and military leaders, including William I, Duke of Aquitaine (c. 763-818), who played a significant role in the struggles between the Carolingians and the Basques.

In the 11th century, the name Willahelm was introduced to England by the Norman conquerors, where it became Anglicized as William. One of the most famous bearers of this name was William the Conqueror (c. 1028-1087), the first Norman king of England, who led the Norman conquest of England in 1066.

The Spanish form Guillermo emerged in the medieval period, particularly in the regions of Spain that were influenced by the Frankish culture and language. It was borne by several Spanish nobles and rulers, including Guillermo I de Bearne (c. 1090-1137), who was the Duke of Aquitaine and Gascony.

Other notable historical figures named Guillermo include Guillermo de Ockham (c. 1285-1347), an English Franciscan friar and philosopher who is renowned for his principle of parsimony, known as Occam's Razor. Guillermo Shakespeare (c. 1564-1616) was an English playwright, poet, and actor, widely regarded as the greatest writer in the English language and the world's most famous dramatist.

Guillermo Tell (c. 1292-1354) was a legendary Swiss folk hero who is celebrated for his defiance against the tyrannical rule of the Habsburg empire. The story of his shooting an apple off his son's head with a crossbow is a well-known folktale. Guillermo de Orange (c. 1533-1584) was a Dutch noble and military leader who led the Dutch Revolt against Spanish rule, earning him the title of "Father of the Fatherland" in the Netherlands.