Green’s Dictionary of Slang

(usu. Aus.) the final assessment, the outcome, the ‘bottom line’.

1899		A. Binstead Houndsditch Day by Day 112: I’m on’y here for a short season while my partner in the Klondyke counts the wash-up.
1953		F. Brown Madball (2019) 118: ‘Hope, though, that we can get Quintana to make it a really clean washup by confessing to the Irby kill’.
2004		R.G. Barrett Rosa Marie’s Baby (2013) [ebook] But the final wash-up with Latte was the icing on the cake.