This article examines the character of Efnisien in the Second Branch of the medieval Welsh collection of stories known as the Mabinogi. From the mid-nineteenth century until the present day, Efnisien has proved a troubling character for critical analysis. A preliminary examination shows that typologically, due to his antagonistic irrationality, he shares traits with both trickster and psychopathic figures. After highlighting these aspects of his characterisation, the article moves on to an analysis of Efnisien’s function in the text. It is observed that Efnisien’s irrationality is incongruous with the contingency and social relevance of the other characters’ actions. He is shown to be the erratic, motivational force within catastrophe, and as such, to personify the inexplicable nature of such life-altering events and lend meaning to uncertain circumstances. From a Žižekian analytic perspective, he functions as a repository figure of ideological excess enabling the rationalization of incomprehensible trauma and securing the fictive narrative in which meaning is produced. Efnisien – trickster, psychopath, figure of excess – is thus shown to be vital to the production of meaning in the Second Branch of the Mabinogi.