Up to Scratch

In the bare-knuckle boxing of the early-to-mid 1800s, a line was literally scratched into the ground at the centre of the ring. At the start of every round, both fighters had to walk up to that scratch unaided. If a fighter was too injured or exhausted to make it to the line under their own power within the allowed time after being knocked down, the fight was over, and they'd lost. Being able to walk up to that mark meant you were fit to continue, hence "up to scratch," which drifted from the boxing ring into a general standard of being good enough for anything.