A “server” is an altar boy (or a seminarian, or, sometimes, a member of a minor clerical order like a subdeacon) who assists the priest celebrating the Catholic Mass.
In Hades, Bloom watches as "A server bearing a brass bucket" precedes the priest into the mortuary chapel, and then listens as the boy sings the Latin lines that the Catholic liturgy prescribes as responses to the priest's: "The server piped the answers in the treble." Stephen has performed these roles in his youth, and in Telemachus he feels that Buck's adoption of the role of a priestly celebrant puts him back in that subordinate position. When Buck leaves his shaving bowl (which has served as a ciborium) behind on the top of the tower, Stephen decides to bring it back down and return it to him: "He went over to it, held it in his hands awhile, feeling its coolness, smelling the clammy slaver of the lather in which the brush was stuck. So I carried the boat of incense then at Clongowes. I am another now and yet the same. A servant too. A server of a servant." Since even the celebrant performs his actions in the service of God, servers may be conceived as doubly indentured, servers of a servant. Thornton observes that a kind of biblical curse may be involved. Ham saw his father Noah's nakedness and all his descendants were cast out: "Cursed be Canaan; a servant of servants shall he be unto his brethren" (emphasis added).
At the conclusion of his pious phase in A Portrait, when the director of Belvedere College asks him whether he feels he has a vocation for the priesthood, Stephen recalls his many fantasies of performing priestly rituals, but
above all it had pleased him to fill the second place in those dim scenes of his imagining. He shrank from the dignity of celebrant because it displeased him to imagine that all the vague pomp should end in his own person or that ritual should assign to him so clear and final an office. He longed for the minor sacred offices, to be vested with the tunicle of subdeacon at high mass, to stand aloof from the altar, forgotten by the people . . . (171-72).
Now, holding the shaving bowl that Mulligan made a chalice, and remembering being an altar boy at Clongowes Wood College, he reflects that his desire for self-effacement seems to have implicated him in another variety of servitude. Just as he once served priests who served God, now he serves Mulligan who serves the British overlord Haines. Though unshackled from the Catholic Church, he is still unfree.