fifth, the, n. — Green’s Dictionary of Slang

fifth, the n.

In phrases

plead the fifth (v.) (also take the fifth) [the Fifth Amendment (1791) to the US Constitution states that no person ‘shall be compelled in any criminal case to be a witness against himself’]

(US) to avoid committing oneself, to refuse to take an action or make a statement.

1956		J. Stearn Sisters of the Night 55: I don’t think he’d plead the Fifth.
1977		A. Brooke Last Toke 156: ‘Take the fifth,’ Richie jived.
1983		C. Heath A-Team 2 (1984) 87: Collins joked, ‘I guess your engine’s pleading the fifth.’.
1990		R. Campbell Sweet La-La Land (1999) 30: You don’t have to say unless you want to say [...] You want to take the fifth with me?
2000		F. Kellerman Stalker (2001) 52: ‘Was that a dig at present company?’ Hayley smiled. ‘I’m taking the fifth.’.