The story
From grace (“elegant movement; poise or balance; charming, pleasing qualities”) + note.
Etymology from Wiktionary, CC BY-SA 4.0.
grace note
Today it means: A musical note, indicated on a score in smaller type with or without a slash through it, played to ornament the melody rather than as part of it. Its note value does not count as part of the total time value of the measure it appears in. Full definition →