Geas

A geis or geas (pl. geasa) is an idiosyncratic taboo, whether of obligation or prohibition, similar to being under a vow or curse, yet the observance of which can also bring power and blessings. The word "geas" is also used to mean specifically a spell prohibiting some action. Geasa occur commonly in Irish and Scottish folklore and mythology, as well as in modern English-language fantasy-fiction.

The word originates in Old Irish, also known as Old Gaelic, and retains the same form in Modern Irish (nominative singular geis /ɟɛʃ/, nominative plural geasa /ˈɟasˠə/; genitive sg. geise /ˈɟɛʃə/, genitive plural geas /ɟasˠ/). In modern Scottish Gaelic, the spelling has evolved in a slightly different direction (nominative singular geas /kʲes/, nominative plural geasan, genitive singular geis or geasa). It has also been borrowed into English in both forms (singular geas or geis /ɡɛʃ/ or /ˈɡi.əʃ/, plural geasa)).