Geas

A ' or ' (pl. ') 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. It is also used to mean specifically a spell prohibiting some action. ' are common 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 '''' , nom. plural '' ''; genitive sg. ' , gen. pl. ' ). In modern Scottish Gaelic, the spelling has evolved in a slightly different direction (nom. sg. ' , nom. pl. ', gen. sg. ' or '). It has also been borrowed into English in both forms (sg. ''geas'' or ''geis'' or , pl. ''geasa'')). Provided by Wikipedia