Green’s Dictionary of Slang

(orig. US) wholly incorrect; often intensified as way off beam.

1942		‘Digg Mee’ ‘Observation Post’ in N.Y. Age 24 Jan. 9/7: King Thomas [...] is right in there with Dot Lytch, but he’d be off his beams, if another would share her dreams.
1949		F. Brown Dead Ringer 40: Unless he’s off the beam mentally, he doesn’t get any actual pleasure out of pulling the trigger. [Ibid.] 122: You’re off the beam there, Cap. It wasn’t any illusion.
1954		‘Nicholas Blake’ Whisper in the Gloom (1959) 46: Never heard of him. You’re off the beam.
1964		J. Ruby quoted in P. Hamill Dirty Laundry n.p.: Do I sound dramatic? Off the beam?
1974		A. Pierrepoint Executioner 94: I made notes [...] and when on the following days I went off-beam, the correction always came in the same words .
1981		S. Berkoff Decadence in Decadence and Other Plays (1985) 16: Don’t look like that / like I’m something just brought in by the cat / OK I’m off the beam tonight.
2003		J. Ellroy ‘Stephanie’ in Destination: Morgue! (2004) 57: LAPD got kickbacks: similar MOs/divergent MOs/MOs off the beam.

(orig. US) right on course, heading in the right direction; thus on the beam in short-cut plays.

1941		Pee Wee Hunt / Casa Loma Orchestra ‘The Boogie-Woogie Man’ 🎵 He’s a leader of a solid ghost boogie-woogie band, / And he makes them play on the beam.
1947		Sun. Post (Lanarks) 1 June 8/4: [headline] For the Love of Mike, Get on the Beam, Mr Shinwell.
1953		J. Thompson Savage Night (1991) 105: You were right on the beam — playing all the angles.
1976		R. Price Blood Brothers 77: I keep alert and on the beam / Because my head is shiny clean.
1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
1997		Simon & Burns Corner (1998) 358: The kitchen boys race to keep up with the crowd at the counter. ‘We on the beam now, ain’t we?’ shouts Gary.