'We is under one o' them big birds,' said Daft Wullie, keeping his eyes averted from the witch's blind stare.
'He means a geas, miss,' said Rob Anybody, glaring at his brother. 'It's like a - '
'- a tremendous obligation that you cannot disobey,' said Miss Treason. 'I ken what a geas is.
In Irish folklore, a geis/geas, pronounced [gɛʃ], can be a taboo, or a positive obligation, or a curse. It's from Irish Gaelic geis, from Proto-Indo-European *gʷhedh- "to beg, wish for".
In English *gʷhedh- became bid "to ask, pray" (bid "to proclaim", as in forbid, is from a different source).
The affixed form *n̥-gʷhedh-to- meaning something like "unwished for" perhaps? (the AHD says "inexorable") became Latin infestus "hostile" and infestāre "to assail, molest", borrowed into English as infest.