si

From Middle English si (“seventh degree or note of Guido of Arezzo's hexachordal scales”), Italian si in the solmization of Guido of Arezzo, from the initials of Latin Sāncte Iohannēs (“Saint John (the Baptist)”) in the lyrics of the scale-ascending hymn Ut queant laxis by Paulus Deacon; thus, also an initialism of Sāncte Iohannēs.

出典: Wiktionary「si」（CC BY-SA 4.0）