Si
来自Big Physics
early 18th century: from the initial letters of Sancte Iohannes, the closing words of a Latin hymn (see solmization).
wiktionary
Acronym of Latin Sancte Ioannes, the phrase ending the hymn Ut queant laxis from earlier words of which the other notes of solfège were derived.
etymonline
si
"yes" in Italian and Spanish; from Latin sic "so" (see sic).
