What is the origin of phrase red nun as a navigation aid?

The phrase "red nun" is not a navigation aid. It's likely you are thinking of "red buoy", a common navigational marker used in waterways.

* Red Buoys indicate the right-hand side of a channel when heading upstream or towards the sea. They are often referred to as "red nuns" because of their shape, which resembles a nun's headwear.

* Green Buoys indicate the left-hand side of a channel when heading upstream or towards the sea.

These color and shape conventions are standardized worldwide to help boaters navigate safely.

It's important to remember that buoyage systems can vary in different countries, so it's always best to consult local charts and regulations before navigating unfamiliar waters.