In Proteus Stephen thinks of the scent of "froggreen wormwood" in Parisian cafes and shortly afterward recalls Kevin Egan "sipping his green fairy as Patrice his white." The references are to absinthe, an intensely alcoholic (as much as 75% ABV) and allegedly hallucinogenic anise-flavored liquor that was popular among artists and writers in Paris in the early 20th century. Later, in Oxen of the Sun, Stephen culminates a long day of drinking with some staggering draughts of absinthe.
For much of the 20th century absinthe had a reputation for mind-altering and mind-damaging properties, and many countries banned its sale soon after the time represented in Ulysses. (France joined the bandwagon in 1914, but the United Kingdom never outlawed the drink, probably because not many people consumed it on the islands.) Most of the charges against absinthe were ridiculously exaggerated, exploited by campaigns of demonization similar to the "reefer madness" campaign that succeeded in outlawing marijuana in the United States. The "wormwood" in the liquor (leaves and flowers of the grand wormwood plant, Artemisia absinthium) has never been shown to be hallucinogenic, addictive, or poisonous, and it may have medicinal properties. Nevertheless, many people at the turn of the 20th century believed that absinthe had powerful effects beyond those of the high alcohol content.
Some absinthes are greenish, because of the presence of botanicals and/or artificial coloring. Others are clear. When mixed with water, botanical essences come out of alcoholic solution to produce a cloudy look called the louche, freeing up some of the herbal flavors in the spirit. Fin de siècle French bars usually served absinthe in this way, dripping water over a sugar cube and into a glass until the alcohol was considerably diluted.
The opalescence probably gave rise to the nickname that absinthe enjoyed in Joyce's time: la fée verte, the green fairy. "Sipping his green fairy as Patrice his white" may imply that Kevin's son drinks the white louche absinthe shown in the photograph (grammatically, "white" must modify "fairy"), or perhaps, as Gifford infers, the reference is to Patrice's lait chaud mentioned earlier in Proteus (with "as" meaning "in the same manner as"). Later in the paragraph about Kevin Egan Stephen thinks, "His breath hangs over our saucestained plates, the green fairy's fang thrusting between his lips."
Yet once more in this paragraph of Proteus, absinthe makes an appearance as someone says or thinks, "Green eyes, I see you. Fang, I feel," apparently connecting the treacherous green fairy to "the green-eyed monster," Iago's description of jealousy in Shakespeare's Othello. Egan has been talking before these two sentences, but his speech concerns young women giving massages to lascivious men. It would be very strange for him to interrupt these thoughts to apostrophize absinthe. Furthermore, it was Stephen who contemplated the fairy's fang thrusting between Egan's lips earlier in the paragraph, and only Stephen has the literary proclivities to connect the green fairy of Paris to Iago's monster. So it seems reasonable to assume that Stephen's internal monologue here breaks into Egan's spoken dialogue. The Shakespearean allusion will return in Circe.
It is probably not an artistic accident that, when the young men leave the maternity hospital for Burke's pub at the end of Oxen of the Sun, just before the hallucinogenic extravagances of Circe, Stephen chooses this drink for his final alcoholic indulgence. After offering to treat everyone to drinks ("Query. Who's astanding this here do? Proud possessor of damnall"), and asking what the others want, he declares his own preference: "Absinthe for me, savvy?" On the second round, all the others join him: "Landlord, landlord, have you good wine, staboo? Hoots, mon, a wee drap to pree. Cut and come again. Right. Boniface! Absinthe the lot. Nos omnes biberimus viridum toxicum diabolus capiat posterioria nostria." Gifford translates Stephen's Latin: "We will all drink green poison, and the devil take the hindmost."
In Circe, Bloom tries to mollify the soldiers by explaining why Stephen is talking so wildly: "He doesn't know what he's saying. Taken a little more than is good for him. Absinthe. Greeneyed monster."