Green’s Dictionary of Slang

a.b.f. n.

[abbr. absolutely bloody final drink]

1. the last drink of a session.

[UK]Partridge DSUE (8th edn) 1/1: from ca. 1915.
sportextreme.com (S.Afr.) 3 May [Internet] No meal is complete without an ABF (absolutely bloody final) drink.
Great Three-Letter Abbreviation Hunt [Internet] ABF – Absolute Bloody Final (drink).
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.

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