Perhaps based on Latin Titanius meaning “of the Titans”. This name was (first?) used by Shakespeare in his comedy A Midsummer Night’s Dream (1595) where it is the name of the queen of the fairies. This is also a moon of Uranus, named after the Shakespearean character.
People with the name Guillelmo have a deep inner desire for travel and adventure, and want to set their own pace in life without being governed by tradition.