fifth column noun

People within a country, etc who sympathize, and will act, with its enemies (expression used by a Spanish insurgent general when four columns were advancing upon Madrid)

fifth columnist /kolˈəm-ist/ noun

fifth generation noun (computing)

The type of computer which incorporates aspects of artificial intelligence

fifthˈ-generātion adjective

Fifthˈ-monarchism noun

Fifthˈ-monarchist noun

Fifth-monarchy men plural noun (historical)

An extreme sect at the time of the Puritan revolution, who looked for a new reign of Christ on earth in succession to Daniel's four great monarchies of Antichrist

fifth wheel noun

1. The spare wheel of a four-wheeled vehicle
2. A coupling on an articulated vehicle that allows the towing of a trailer
3. A superfluous or useless person or thing (informal)

plead or take the Fifth (US informal)

To refuse to testify, in accordance with the Fifth Amendment