In fact we have two quite distinct interpretations of the name Tulkas from different periods of Tolkien's writing. In the very early Qenya Lexicon, the name is said to be Elvish in origin, deriving from a root tuluk meaning 'strong' or 'steadfast'. However, in the notes for the essay Quendi and Eldar, written decades later, the name is derived from a quite different source, originating in the language of the Valar. In that tongue Tulkas' original name was Tulukhastāz, 'golden-haired'. As by far the later of the two sources, the interpretation from Quendi and Eldar (given in volume XI of The History of Middle-earth) is preferred here.