Longman Dictionary of Contemporary English
Wiktionary
n. 1 A musician who plays several musical instruments at once. 2 (context idiomatic by extension English) An organisation or business that is effectively run by only one person.
Wikipedia
A one-man band or one-woman band is a musician who plays a number of instruments simultaneously using their hands, feet, limbs, and various mechanical and/or electronic contraptions. One person band musicians also often sing while they perform. The simplest type of "one-person band" — a singer accompanying herself on acoustic guitar and playing a harmonica mounted in a metal "harp rack" below the mouth — is often used by buskers and folk music singer-guitarists. More complicated setups may include wind instruments strapped around the neck, a large bass drum mounted on the musician's back with a beater which is connected to a foot pedal, cymbals strapped between the knees or triggered by a pedal mechanism, tambourines and maracas tied to the limbs, and a stringed instrument strapped over the shoulders (e.g., a banjo, ukulele or guitar). Since the development of Musical Instrument Digital Interface (MIDI) in the 1980s, musicians have also incorporated chest-mounted MIDI drum pads, foot-mounted electronic drum triggers, and electronic pedal keyboards into their set-ups.