When the whores in Bella Cohen’s brothel hear that Stephen is “back from Paris” (15.3873) they ask him to give them “some parleyvoo”. Stephen obliges with two pieces of rambling monologue and ends:
“also if desire act awfully bestial butcher's boy pollutes in warm veal liver or omlet on the belly pièce de Shakespeare.” (15.3908-09)
This provokes Bella Cohen’s mirth:
(Clapping her belly sinks back on the sofa, with a shout of laughter.) An omelette on the... Ho! ho! ho! ho!... omelette on the... (15.3911-12)
What reads like a figment of Stephen’s drunken imagination seems to have been some special treat to help worn-out customers regain some of their sexual prowess.
First we find in Léo Taxil’s La prostitution contemporaine: étude d'une question sociale (1884), p. 166:1
Parmi les pratiques ridicules, il faut citer celle des clients qui se font appliquer une omelette brûlante sur le ventre où à la chute des reins.
[Translation: Amongst other ridiculous practices is one where [a prostitute’s] client has a burning-hot omelette applied to his belly or to the small of his back.]
Joyce refers to a variation in which the omelette is applied to the belly, but in another variation it is applied to the small of the back, as evidenced slightly earlier in print, in the Dictionnaire érotique modern by “un professure de langue verte [i.e. Alfred Delvau]” (1864), p. 176:
Jeu de l’omelette (le). Que les filles de bordel jouent spécialement avec les vieux libertins qui voudraient - et ne peuvent — les baiser. On fait une omelette, et, dès qu'elle est brûlante, On l'applique aussitôt sur son vieux cul ridé.
[Translation: The omelette game. This is one which the girls of the brothel play especially with old libertines who want to — but cannot — f—k them. You make an omelette, and, as soon as it is burning hot, you apply it straightaway onto his wrinkled old ass.]
Earlier still, a coloured French lithograph from around 1820-30, published as one of a series of twelve under the title Journal des connaissances utiles dédié au beau sexe (A Paris Rue des Déchargeurs) is headed “Reméde contre l’Usure” and has the caption “Une Omlette chaude & vous banderez”.2 It shows a gentleman about to have intercourse with a prostitute while another girl drops an omelette from a frying-pan on his naked backside (or the small of his back).
I am grateful to Aida Yared for drawing my attention to the Dictionnaire érotique modern.