From Middle Dutch brac (whence Dutch brak). More at brackish.
Meanings
(obsolete, uncountable) Salty or brackish water.
The very earth to fill the hungry mawe;
When they far'd best, they fed on Fearne and brack, 1627, Michaell [i.e., Michael] Drayton, “The Moone-calfe”, in The Battaile of Agincourt. […], London: […] [Augustine Matthews] for William Lee, […], →OCLC:
An opening caused by the parting of a solid body; a crack or breach.
You may find time out in eternity,
Deceit and violence in heavenly Justice,
Life in the grave, and death among the blessed,
Ere stain or brack in her sweet reputation. 1624 June 6 (licensing date; Gregorian calendar), John Fletcher, “A Wife for a Moneth”, in Comedies and Tragedies […], London: […] Humphrey Robinson, […], and for Humphrey Moseley […], published 1647, →OCLC, Act I, scene i:
A flaw in cloth.
[…] You must take care that all the bracks and rents in the Linen be duly mended. 1677, Hannah Woolley, The Compleat Servant-Maid, London: T. Passinger, page 164:
MAIRE MIRKIN (played by Brid Brennan): ”Sit down please the both of you. You’ve intruded right into the middle of our nightcap, young man. I was just about to serve a toddy to Hector and myself. Can I fix you one?”.
ARM (played by Cosmo Jarvis): A wha…?
HECTOR: Do, dear, yeah. And, um (clicks tongue) cut us a few wodges of brack while you’re at it. 2020 January 6, Joe Murtagh, Colin Barrett, 01:18:44 from the start, in Calm With Horses (film), spoken by Hector (David Wilmott):