Green’s Dictionary of Slang

1. the last drink of a session.

1984		Partridge DSUE (8th edn) 1/1: from ca. 1915.
2002		sportextreme.com (S.Afr.) 3 May 🌐 No meal is complete without an ABF (absolutely bloody final) drink.
2002		Great Three-Letter Abbreviation Hunt 🌐 ABF – Absolute Bloody Final (drink).
2007		A. Drysdale My Neighbour Madiba 80: Paying good money to yet another, and another club we slumped at last for an ABF, an Absolutely Bloody Final.

2. used in non-drinking context.

2010

P. Brown Close to Evil 5: I started going through the mail, only ABF’s—‘Absolutely Bloody Final’ demands for payment-meriting attention.