Choose the correct meaning of the following idiom:

A turn coat

One who changes opinions

The correct answer is one who changes opinions. The idiom "a turncoat" is a derogatory term for someone who changes their allegiance or beliefs, especially for reasons of expediency. The term is thought to have originated from the practice of wearing a coat with a badge or pin on the sleeve to signify one's political or religious affiliation. If someone "turned their coat," they would literally turn the coat inside out to show their allegiance to the opposing side.

The other options are incorrect. "A wet coat" is simply a coat that is wet. "A poor man" is someone who does not have much money. "A man of principles" is someone who adheres to a set of moral or ethical principles.