The story
From sol and fa, two of the sung syllables.
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: a method of sight singing music that uses the syllables do (originally ut), re, mi, fa, sol (or so), la, and si (or ti) to represent the pitches of the scale, most commonly the major scale. The fixed-do system uses do for C, and the movable-do system uses do for whatever key the melody uses (thus B is do if the piece is in the key of B). Full definition →
From sol and fa, two of the sung syllables.
Etymology from Wiktionary, CC BY-SA 4.0.