The word is from Irish folklore, in which a geis could be a prohibition or taboo, a positive injunction or obligation, something unlawful or forbidden, a curse, or a spell or incantation. To violate one led to misfortune and death. In the heroic legends of Ireland, this is what happened to the hero Cuchulain and to Conaire Mor, the King of Ireland, both of whom were unable to avoid breaking their geasa (the Irish plural of geis). The latter had many geasa imposed on him, such as never to sleep in a house from which firelight could be seen after sunset and never to be away from his capital at Tara for more than nine nights at a time.
In Scots Gaelic it’s spelled geas, as it can also be in modern Irish; historically there have been other spellings. Saying it is a tricky problem because Irish spelling and pronunciation is rather difficult for non-natives. Both geis and geas are said as (ɡɛʃ ) and the plural as (/ɡɛʃə/).
Few dictionaries I have here include it. That’s strange, since the word is a staple of modern fantasy books, more commonly in the geas spelling. It seems sometimes you can’t pick up a work of that genre without finding someone is under the influence of a geis. Its major sense in fantasy is magical, referring to a spell or enchantment. It would be easy to quote a couple of hundred examples, but two will suffice. Jack L Chalker wrote in A Jungle of Stars: “The Pull began, that ancient geas laid upon him in times past by The Race, that curse that tied him to his planetary sphere.” More weirdly, Charles Stross has a character say in The Jennifer Morgue, “You’re looking at a hardware circuit designed to implement a love geas using vodoun protocols and a modified Jellinek-Wirth geometry engine.”
Terry Pratchett employs it in two of his fantasy works. To confuse us further about the way to pronounce it, characters in both books say it (or mishear it) as geese. In A Hat Full of Sky two characters exchange a snatch of Scots-style conversation: “‘Tis a heavy thing, tae be under a geas.’ ‘Well, they’re big birds,’ said Daft Wullie.”