Encyclopaedia of the Celts: Garwen - Geoffrey of Monmouth

It is a geis to the Ulster champion, Fergus mac Roy, that he must not refuse an invitation to a feast; on this turns the Tragedy of the Sons of Usnach. It is not at all clear who imposed these geise or how any one found out what his personal geise were - all that was doubtless an affair of the Druids. But they were regarded as sacred obligations, and the worst misfortunes were to be apprehended from breaking them. Originally, no doubt, they were regarded as a means of keeping oneself in proper relations with the other world - the world of Faery - and were akin to the well-known Polynesian practice of the 'tabu.' Rolleston prefer, however, to retain the Irish word as the only fitting one for the Irish practice.

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