Green’s Dictionary of Slang

loon (about/off) v.

[loon n.1 ]

to act crazily or irresponsibly; as n., a wild time.

[UK]Oz 4 25: Just off for a love loon with Lucy in the Sky with Diamonds.
[UK]Oz 7 26/2: I’m just looning off to Mr Fish to pick up my caftan. Then I’m looning off to pick up my bell from Asprey.
[Can]J. Mandelkau Buttons 64: There were about thirty Angels looning about in the front room.
[UK]M. Novotny Kings Road 96: You stay around and have a good loon [...] the party’s just beginning.
[US]A. Carter in Michaels & Ricks ‘The Language of Sisterhood’ in State of the Lang. 233: The hippy penchant for play survives in the way sisters enjoy [...] ‘looning’ together, that is, dancing and clowning around for fun.

In compounds

loon pants (n.) (also loons)

trousers with enormous flared bottoms, esp. beloved of early 1970s hippies.

[UK]Times 5 Dec. 15: Dress manufacturers [...] are in despair at the number of young customers in loon pants and T-shirts.
[Ire]P. McCabe Breakfast on Pluto 72: A little puddle [...] ticking in small drops on to my stinky, balding velvet loons.
[UK]Guardian 23 July 10: A pair of his velveteen ‘loon’ pants had failed to reach it’s reserve price in a Sotheby’s sale of rock memorabilia.
[UK]Guardian G2 4 Nov. 5: We wore 21-inch loons. For the uninitiated, loons were trousers and the 21 inches were the circumference of their flare.
[UK]Observer Rev. 13 Feb. 8: There’s Keith Arnatt in loons.