Green’s Dictionary of Slang

1. nonsense, rubbish.

1841		W.J. Neale Paul Periwinkle 614: I hope that’s finished you! You thrice-dyed incarnation of lies and rhubarb!
1849		Paul Pry 7 May 6/1: Mr. W—d, the dentist [...] not to boast so much about the innumerable cases in which he has administered chloroform and ether. They will not do in all instances, and ‘rhubarb is rhubarb, call it what you will’.
1926		Wood & Goddard Dict. Amer. Sl. 39: piffle. Nonsense, twaddle, applesauce, stewed rhubarb.
1946		H.A. Smith Rhubarb 21: ‘Rhubarb!’ He spoke sharply.
1959		H.E. Bates A Breath of French Air (1985) 183: ‘I like to pay.’ ‘Rhubarb!’ Charley said. ‘This one’s on me!’.
1966		P. Larkin letter 21 May in Thwaite Sel. Letters (1992) 384: Rhubarb, rhubarb. See you before long.
1975		D. Nobbs Fall and Rise of Reginald Perrin (1976) 136: General chit chat. Rhubarb rhubarb. Introduce subject of madness tactfully.
1989		S. Armitage ‘Canard’ in Zoom 54: Poetry was rhubarb.

2. (orig. US) an argument, a noisy dispute, esp. one that takes place on the field of play at a sporting event.

1943		Amer. N&Q Dec. 134/2: Rhubarb [...] is ‘squabbling, scuffling, wrangling’.
1947		Time 22 Sept. 70: Next inning, at the plate, there was a face-to-face exchange of hot words between Robinson and Garagiola—the kind of rough passage that fans appreciatively call a ‘rhubarb’.
1952–3		Mad mag. Dec.–Jan. 6: There’s a big rhubarb going on. The umpire is examining the ball!
1953		W. Harrison Army Girl (1962) 61: You’re making a big thing of it [...] I really don’t rate all the rhubarb.
1964		R. Helmer Stag Party 135: ‘My heart bleeds for Mr Hammond.’ ‘Heard about your rhubarb with him.’.
1975		L. Rosten Dear ‘Herm’ 163: Well, that made Flo blow a gasket and before you know it we are having a slam-bang rubarb.
1987		T. Wolfe Bonfire of the Vanities 27: A riot. [...] Shouts . . . chaos . . . a real rhubarb.