noun
plural fifths
[singular] :number five in a series
[count] :one of five equal parts of something
[count] :a unit of measure for alcoholic liquor that is equal to one fifth of a U.S. gallon (approximately 750 ml) also; :a bottle that holds this amount of liquor
take/plead the Fifth
chiefly US :to refuse to answer questions in a court of law because your answers might be harmful to you or might show that you have committed a crime
The phrase take/plead the Fifth refers to the Fifth Amendment of the U.S. Constitution, which says that citizens of the U.S. cannot be required to give testimony that could be used against them in a court of law.
adjective
occupying the number five position in a series
US