Meaning

noun the syllable naming the fourth (subdominant) note of the diatonic scale in solmization

originFrom Glover's solmization, from Middle English fa (“fourth degree or note of Guido of Arezzo's hexachordal scales”), Italian fa in the solmization of Guido of Arezzo, from the first syllable of Latin famulī (“servants”) in the lyrics of the scale-ascending hymn Ut queant laxis by Paulus Deacon.

Etymology from Wiktionary, CC BY-SA 3.0