The simple past form of the verb ir in Spanish is the same as that of the verb ser. Is it true that fui of ir is derived from fui of esse (>ser), as many claim? From the historical viewpoint, in many romance languages, it is common that the action verbs such as andare and vadere share the conjugation of ire (>ir). Therefore, it is possible to assume that any action verbs would be affected by the verb ir. In addition, we also accept the possibility of intervention of another action verb fugĕre on the basis of the analysis by Ferreira Strogenski (2013). Last, if we accept this theory, then we will be able to explain why phonologically the f- is generated in the simple past form of ir. Grounded on these arguments, this article discusses that the etymology of the simple past form fui of ir is not derived from fui of esse, but from fŭgi of fugĕre.