The story
From note + bash, as an allusion to bashing the keys of a piano in an unmusical fashion when first learning a piece.
Etymology from Wiktionary, CC BY-SA 4.0.
note-bash
Today it means: To learn the most basic aspects of a piece of music (such as the notes and rhythm) via rote repetition, without regard for tempo, dynamics, style or musicality; typically done when first learning a new piece, as a way to understand it and build muscle memory. Full definition →