A number of the vernacular texts stemming from the reign of Alfonso X (king of Castile and León from 1252-1284) discuss the numerous purported etymologies of individual words. This study investigates these explanations in terms of their place in the history of etymology, their use of sources, their formal structure, their attitude to phonological and semantic change, their function in the respective text, and the light they shed on the ""authors'"" notions of what a word actually is. The study is based on a corpus of over 1,000 such etymological explanations.