Melismatic singing was held by Christian mystics to be the highest form of religious utterance: “It is a certain sound of joy without words,” St. Augustine wrote of melismatic chanting in the fourth century, “the expression of a mind poured forth in joy.”1 It came to be called jubilated singing, after jubilus, Latin for a “call” upon God (as in Charlemagne’s Admonitio, quoted earlier; compare the root ju-, pronounced “yoo,” as in “yoo-hoo!”). This musical jubilation, in fact, was the means through which the Latin word took on its secondary (in English borrowings, primary) association with joy.

Taruskin, Richard. “Music from the Earliest Notations to the Sixteenth Century, Volume 1 of The Oxford History of Western Music.” (2005).
Notes:

1. Jacques Paul Migne, ed., Patrologiae cursus completus, Series Latina, Vol. XXXVII (Paris, 1853), p. 1953, trans. Gustave Reese in Music of the Middle Ages (New York: Norton, 1940), p. 64.