13Ó hÓgáin, Dáithí. Fionn Mac Cumhaill: Images of the Gaelic Hero. Dublin: Gill and Macmillan, 1988. 3-4.
The action of Fionn is sometimes described as "biting" rather than "sucking" the thumb to gain his special knowledge. According to Ó hÓgáin (p. 52), "The magical knowledge of seers and poets was known as fios, which is the root used to designate Fionn's gift, as well as to describe his celebrated 'thumb of knowledge.'"