i) A binding command or obligation, so deeply rooted in custom, tradition and The Way We Do Things, that it would be unthinkeable to refuse to go where it obliges you to go and do what it obliges you to do. Magic is also generally involved in some fashion. On a world where all concepts have polar opposites, the opposite of a taboo.
2) A kind of comical-looking bird looking like a cross between a goose and a dodo, possibly called into being by residual magic looking for an outlet after the departure of the Sourcerer and in accordance with Rincewind's stubborn belief that a geas is a kind of avian lifeform...
Roundworld has Geas too: The observing of one's geasa is believed to bring power and good fortune. Often it is women who place geasa upon men. In some cases the woman turns out to be goddess or other sovereignty figure.
The geis is often a key device in hero tales, such as that of Cúchulainn in Irish mythology. Traditionally, the doom of the hero comes about due to their violation of their geis, either by accident, or by having multiple geasa and then being placed in a position where they have no option but to violate one geis in order to maintain another. For instance, Cúchulainn has a geis to never eat dog meat, and he is also bound by a geas to eat any food offered to him by a woman. When a hag (actually the trickster-goddess Morrigan in disguise) offers him dog meat, he has no way to emerge from the situation unscathed; this leads to his death. The story also vividly describes how Headology operates in the mythology of our world - it becomes clearer that the geas operates according to the laws of headology. (If you believe something to be so, it is more than halfway to being so).