Turncoat (noun)

1. A person who betrays their party, cause, or belief by switching to an opposing one.
2. A person who changes sides during a conflict or war.

Origin:

From turn ("to change one"s allegiance") + coat ("clothing, appearance"), from middle english turnecote, equivalent to turn + coat.

Examples:

1. He was considered a turncoat for betraying his country.
2. The politician was accused of being a turncoat for switching parties.
3. The turncoat's defection was a blow to the organization.
4. The turncoat's actions were met with disdain by his former comrades.
5. The turncoat's betrayal was unforgivable.