Green’s Dictionary of Slang

1. a musician, usu. in a rock band.

1977		K. Gilbert Living Black 162: I used to be a muso and a hustler from the city.
1983		J. Byrell (con. 1959) Up the Cross 8: Mick the Muso looked like what he was — a musician.
1986		C. Bowles G’DAY 25: A muso plays gigs and mainlines.
1997		L. Davies Candy 73: Tucker was a muso who sometimes bought our smack.
2000		Guardian Rev. 28 Jan. 13: He’s a muso.
2003		McGill Reed Dict. of N.Z. Sl.
2007		L. Redhead Cherry Pie [ebook] I dashed toward the van, pushed past the muso [...] and lay flat on my back behind a PA.
2011		IOL News (Western Cape) 23 Mar. 🌐 With a mixture of musos it is definitely going to be something different and interesting.
2013		D. Whish-Wilson Zero at the Bone [ebook] Musos and their hangers-on was a good new market for him.

2. attrib. use of sense 1.

1986		T. Winton That Eye, The Sky 61: The house was full of muso types and people doing drugs.
2000		N. Barlay Crumple Zone 29: This is Musoland, one of those mythical places where young musicians go to get hip and take off.