Green’s Dictionary of Slang

on the line phr.

[gambling use]

1. [late 19C+] honest, straightforward, in the open.

2. [20C+] of money, either put at stake or, e.g. in the case of a drug deal, advanced as a loan.

3. [1940s–50s] (US) prepared, in the offing.

4. [1970s] under interrogation.

5. [1980s] (Aus.) under police observation.

6. [2000s] (US prison) for sale.

In phrases

put one’s ass on the line (v.) (also put one’s balls on the line) [ass n. (4)/balls n. (3)]

[1980s+] (US) to put oneself into a position of responsibility, to take risks; to face punishment.

put someone on the line (v.)

[1990s+] to place someone in a difficult or challenging position.