late 13c., "musical instrument," from Old French instrument "means, device; musical instrument" (14c., earlier estrument, 13c.) and directly from Latin instrumentem "a tool, apparatus, furniture, dress, document," from instruere "arrange, furnish" (see instruct). Meaning "tool, implement, utensil" is early 14c. in English; meaning "written document by which formal expression is given to a legal act" is from early 15c.
instrument in·stru·ment (ĭn'strə-mənt)
A tool or implement, as for surgery.