on the line phr.
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.
[1980s+] (US) to put oneself into a position of responsibility, to take risks; to face punishment.
[1990s+] to place someone in a difficult or challenging position.