Green’s Dictionary of Slang

1. to spend or waste money [opposite of clean up v. (1)].

1909

W. Irwin Confessions of a Con Man 14: I cleaned up thousands [...] in those thirty years – and mussed them up as fast as I’d cleaned them.

2. (US) to treat roughly, to beat up.

[1866		Night Side of N.Y. 47: Wine and women, you know – smashed a mirror or two – mussed up things generally].
1912		Day Book (Chicago) 13 Nov. 13/2: George Carpentier [...] who was horribly mussed up by Billy Papke, will display his prowess among the light heavyweights.
1919		N.Y. Tribune 21 Sept. 3/3: The very demonstrative friend who is always mussing up somebody.
1931		Philadelphia Inquirier (PA) Mag. 18 Jan. 11/4: ‘When I connects wit’ yer map, yer dial is apt to git mussed up — ’.
1959		‘Paul Merchant’ ‘Sex Gang’ in Pulling a Train’ (2012) [ebook] We don’t take to any man mussing up one of our girls.
1999		J. Maple Crime Fighter 216: [W]e had to be vigilant to prevent other officers from ‘mussing up’ our own prisoners.