Green’s Dictionary of Slang

1. honest, straightforward, in the open.

1884		Ally Sloper’s Half Holiday 3 May 2/1: As an ‘on the line’ man said to me—‘Mima, dea— I mean, Miss Sloper’.
1902		Bulletin (Sydney) 13 Sept. 35/2: Oh, I suppose they’d have to be a woman – if only to keep the doctor on the line.
1944		D. Runyon Runyon à la Carte 88: You will tell her to get it on the line at once and save herself trouble.
1959		E. De Roo Go, Man, Go! 105: Pa wanted it on the line. He was keeping Paul awake to have it out.
1973		P. Maas Serpico 180: Once he had remarked that he had heard that things like this were going on, but never in this way, never directly from a cop laying it on the line as Serpico was.
1981		T. Wilkinson Down and Out 131: If what you’re saying is really on the line and straight, then there’s no reason why they can’t review your case.

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

1900		Sporting Times 9 June 1/5: I’m the P.C. with landscapes on the line, I am. You surely don’t expect a R’yal Academician to ruin his hands by mixin’ up in street scraps, do ye?
1929		Hostetter & Beesley It’s a Racket! 233: on the line—Immediately; ready and waiting; e.g. ‘He put down $150 on the line.’ NOTE—A term borrowed from aviation and used on landing fields to indicate a plane that is ready to take off.
1931		D. Runyon ‘Gentlemen, the King!’ in Runyon on Broadway (1954) 174: He [...] will lay twenty-five G’s on the line.
1940		F.S. Fitzgerald ‘A Man in the Way’ in Pat Hobby Stories (1967) 37: I hate to give an idea without money on the line.
1957		H. Simmons Corner Boy 44: Wasn’t many guys Monk would let have junk without cash on the line.
1981		J. Ellroy Brown’s Requiem 12: I [...] told him to meet me at my place in an hour and to bring coffee—there was work on the line.
1991		B. Hamper Rivethead (1992) 152: The problem was Eddie simply got the yips whenever money was on the line.

3. (US) prepared, in the offing.

1962

P. Crump Burn, Killer, Burn! 48: Everytime I get a lay on the line, something messes it up.

4. (US black) at risk, in danger.

1968		G. Radano Walking the Beat 97: ‘Everybody wants to judge us these days! [...] What the hell do they know about a mob? No bastard knows anything except the poor cop whose ass is on the line’.
1970		G. Cain Blueschild Baby 93: Another near fatal O.D., but always I return from the dead to try again, my life is always on the line.
2000		W.D. Myers 145th Street ‘Monkeyman’ in 77: [T]he truth was that I was glad it was Monkeyman on the line, and not me.

5. under interrogation.

1974

G.F. Newman You Flash Bastard 124: Even an important QC, a judge almost, cringed when on the line.

6. (Aus.) under police observation.

1989

J. Morton Lowspeak.

7. (US prison) for sale.

2000

Other Side of the Wall: Prisoner’s Dict. July 🌐 On the Line: For sale (L.A. County Jail).