In a book chock-full of neologistic compounds, one stands alone: "contransmagnificandjewbangtantiality." Stephen elaborates wildly on the Nicene "consubstantiality" opposed by Arius, to produce a word that has elicited much comment. Joseph Campbell, in particular, has inspired many people to read the word as a key to the novel's theological mysteries, conveying eastern religious ideas of the radical immanence of divinity and the mystical unity of subject and object. But popular nonsensical versions of the word long antedate Joyce, calling into question such ambitious readings.
Between con- and -substantiality intrudes the prefix "trans." This may be a reference to Arius' belief that the Father "made" the Son, rather than "begetting" him as the Nicene Creed would have it. In Arian doctrine the Father is essentially different from, and superior to, the Son. He transcends him—is transubstantial rather than consubstantial.
Alternatively, following Campbell, "trans" might be read as referring to transubstantiation, the Catholic doctrine that bread and wine become miraculously changed into the body and blood of Christ during the sacrament of the Eucharist. By this reading, "trans" and "con" would not negate one another as Arian and Nicene Christology do. Consubstantiality and transubstantiation both involve sharing divine substance (between Father and Son, and between Christ and the eucharistic meal), so the prefixes would intensify one another, pointing toward the novel's preoccupation with metempsychosis, transpersonal identification, and the coincidence of opposites.
The next segment, "magnific," recalls the Magnificat, a very ancient church hymn based on the words that the Virgin Mary utters when Elizabeth says, "Blessed art thou among women, and blessed is the fruit of thy womb": "And Mary said, My soul doth magnify the Lord, And my spirit hath rejoiced in God my Saviour. For he hath regarded the low estate of his handmaiden: for, behold, from henceforth all generations shall call me blessed. For he that is mighty hath done to me great things; and holy is his name" (Luke 1:42, 46-49). In Latin, magnificare means to esteem or prize highly. When Mary says that her soul magnifies the Lord, she evidently means something like "manifests his greatness"—it is God's greatness, not her own, that she celebrates.
But one could infer that Mary has become magnificent, as she certainly did in later Catholic practice. For Campbell, "My soul doth magnify the Lord" signifies that "God is within me," and this refers "not solely to the condition of the Mother of God, two thousand years ago in Judea, but to every one of us, here and now" (Mythic World, Modern Worlds, 80). Magnificat signifies the interpenetration of self and other proclaimed in the Sanskrit saying "tat tvam asi" (thou art that). Divinity, argues Campbell, does not lie outside of human nature and history; it is radically immanent in all people at all times.
The implications of the next word, "jew," seem relatively straightforward. Jesus was a Jew. When God took on human flesh, he came here as a Jew. And yet he was rejected by his own people, leading many Christians through the ages to accuse the Jews of killing their Lord. Leopold Bloom walks through the novel as the object of such anti-Semitic suspicions, but he demonstrates Christ-like qualities of generosity and compassion and offers spiritual assistance to Stephen. Although Stephen cannot be thinking of Bloom when he coins his big word, it may anticipate the union of the two men, who as Father-Son and "jewgreek" exemplify the kinds of union of opposites implied by Campbell's tat tvam asi.
By contrast, the word "bang" seems open to various interpretations. Sticking narrowly to the theological disputes that Stephen is pondering, Gifford says that it "suggests both the controversial origin of Christianity and the sustained controversy over Arianism." But the association of God with explosive power could also support Campbell's idea of immanent divinity. In Nestor, when Stephen is confronted with the headmaster's traditional teleological idea of "the manifestation of God" at the end of time, he points toward the hubbub of the hockey game and says, "That is God . . . A shout in the street." For Stephen, creative and destructive power is immanent in all things. He is terrified by a thunderclap in Oxen of the Sun because, like his creator Joyce, he hears the voice of God in thunder. (Another possibility, given current slang, would be to hear a reference to the divine insemination of Mary. But I do not know of any evidence that this sexual idiom would have been available to Joyce.)
Stitching together some of the variant readings offered in this note, one could conclude that "contransmagnificandjewbangtantiality" represents simply a condensation of some arcane early Christology, or that it compresses essential elements of the novel's post-Christian metaphysics.
 But commentators have also found various forms of a popular 19th century jaw-breaker that must have been the genesis of Joyce's word. Rather like the late-20th century "supercalifragilisticexpialidocious," words like "transmagnificanbandancial" and "transmagnificandubandanciality" originated at least as early as the 1830s, possibly in America, and survived and evolved for many decades. In a short survey article on James Joyce Online Notes John Simpson suggests that Joyce could have encountered the word either in the writings of the early 19th century Irish poet James Clarence Mangan, whom he revered, or in popular contemporary venues like spelling bees, songs, and pantomimes. Joyce grafted the Christian "consubstantiality" onto this existing rootstock, and he may well have found various kinds of theological significance inherent within it, but the word's silly origins should perhaps discourage excessively solemn interpretations.