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