go off the boil

verb4

1. •
   (UK, Australia) To cease to boil when heat is no longer applied.

   • All puddings must be boiled in plenty of water, turned frequently, kept closely covered, and never allowed to go off the boil. — 1891
2. •
   (UK, Australia) To lose interest; to pall.

   • As one of the rearguard put it, "We laid up until the Hun had gone off the boil a bit and slipped out the following night." — 1955
3. •
   (UK, Australia) To diminish in intensity or urgency.
4. •
   (UK, Australia) To become less successful.