looney tune, n. — Green’s Dictionary of Slang

looney tune n.

also looney tunes, loony tune

[Looney Tunes, the series of film cartoons created for Warner Bros. by the team of Hollywood animators Hanna-Barbera and released 1930–69, then shown on TV from 1950s; the term was popularized by another Hollywood figure, Ronald Reagan, to describe such figures as Libyan leader Colonel Gaddafi]

1. a crazy person, a lunatic.

1957		H. Ellison ‘Kid Killer’ in Deadly Streets (1983) 110: The other boys had mothers, but he had a banana! A looney tune.
1979		E. Torres After Hours 206: He was the local looney tunes.
1987		Time 7 Dec. 66: ‘There’s some loony tunes out there,’ he says.
1998		D. Hecht Skull Session 205: Rizal glanced at him [...] a real looney-tune.
1998		P. Cornwell Point of Origin (1999) 179: Unless you’re dealing with a loony tune, you don’t find killers mutilating the faces of victims.
2001		C. Brookmyre A Big Boy Did It And Ran Away 63: No paymaster on that occasion, no Looney-Tunes collectively trumpeting their responsibility.

2. a cartoon.

1977

(con. 1969) M. Herr Dispatches 60: It was like turning up in the middle of some black looneytune where the Duck had all the lines.