A term often used in Irish literature to imply a magical demand or prohibition placed on a celebrated personage. The word itself has been variously explained. One theory relates it to ‘guidhe’, meaning request or prayer, while another theory is that in origin it meant ‘happen ing’ or ‘destined occurrence’. It frequently occurs in its plural 'gessa' (later, ‘geasa’).
The idea seems to have been central to ancient Irish ritual of kingship, the king being a sacred person whose function it was to maintain equilibrium in the society and in the environ ment. The life of such a person would be hedged about by magic, and he therefore had to always make the correct decisions relative to his func tion. The phenomenon of ‘geis’ specified what his approach should be. The concept of ‘geis’ prob ably originated from an idea that the goddess of sovereignty imposed certain conditions on her kingly spouse, and it is significant that the early literature focusses in particular on the varieties of ‘geis’ associated with kings of the great cultic cen tre, Tara. From this, it spread to the contexts of great heroes also, and is instanced in the cases of Cu Chulainn and Fionn mac Cumhaill, among others. In all events, the result of acting contrary to ‘geis’ is the social destruction and the personal death of the individual.
Probably as a reflex from the image of female sovereignty, the imposition of ‘geis’ was represented in lovestories as being a special privilege of women (see, for instance Deirdre, and Grainne in the entry on Diarmaid ua Duibhne). In the later romances and in folklore such con ditions became known as ‘heavy geasa of magic', and they were much employed as a device to set a plot in motion. A hero may lose to a hag at chess or cards, and as her prize she imposes ‘geasa’ on him to perform some seemingly impossible task.
RIA Dictionary s. v. ‘geis’; J R Reinhard (1933); Philip O’Leary in Celtica 20. 85107; David Greene in Hans BekkerNielsen ed (1977), 919; E P Hamp in Eriu 32, 1612.