Barbara Freitag devotes a chapter to the etymology of the name in her book ‘Sheela-Na-Gigs: Unravelling an Enigma.’ She documents references earlier than 1840, including a Royal Navy ship Sheela Na Gig, and an 18th-century dance called the Sheela na gig. The Irish slip jig, first published as “The Irish Pot Stick” (c.1758), appears as “Shilling a Gig” in Brysson’s A Curious Collection of Favourite Tunes (1791) and “Sheela na Gigg” in Hime’s 48 Original Irish Dances (c.1795). These are the oldest recorded references to the name, but do not apply to the architectural figures. The Royal Navy’s records indicate the name of the ship refers to an “Irish female sprite”.Freitag discovered that “gig” was a Northern English slang word for a woman’s genitals.[9] A similar word in modern Irish slang gigh also exists, further confusing the possible origin of the name.
[If the term is pronounced ‘sheela-na-jig’, the origin seems more easy to apprehend. Instead of indulging in acrobatics, many post-Romanesque figures are depicted as dancing, sometimes with a hand raised as in a jig. This was a northern English dance considered lewd at the time (like the volta or jumping-dance later). It was imported into Scotland as the Scottish Jig, écossaise, Schottishe, and to Ireland as the Irish Jig – the most lewd of its forms. The rhythm remained popular, and the more seemly gigue (soft French g) was a musical form employed by Byrd, Bull, Purcell, Marais, etc.
Jig as a verb had strong sexual connotations, as did Jazz/Jizz much later. Compare the related verb to jag which means to enter sharply. Consider also the verb to jog.Compare also the old word giglot (lewd and wanton woman) and the contemporary gigolo (a kind of male prostitute). The term ‘jiggery-pokery’ was applied to rapid or desperate sexual liaisons. ‘Jig-a-jig’ is a term which survives to this day in West Africa, from the sailors in days of yore and today; and ‘to jiggle’ was Victorian slang for ‘to fuck’. Thus the post-Romanesque figures carry on the Romanesque condemnation music, dance and entertainment thought (by the English) to be particularly riotous in Ireland.