“In the doldrums” began as a sailing term for a calm equatorial zone. Over time, it came to mean feeling low, stuck, or inactive.

“Cut and run” means to leave quickly, often to avoid trouble. The phrase comes from sailing, when ships cut anchor cables to escape fast.

“Dead as a doornail” is a very old English phrase. It likely grew from the way door nails were hammered flat and made unusable.