Green’s Dictionary of Slang

to deride, to dismiss.

1909		C.B. Chrysler White Slavery 70: No ‘dick’ has anything on you. And if you’re you get nicked for ‘laughing’ you can ‘raspberry’ the ‘bull’ and get ‘sprung’.
1942		P. Larkin letter 19 Sept. in Thwaite Sel. Letters (1992) 45: If I’d composed a poem like that one I’d keep it jolly dark, my God I would. Ought to have been duly raspberried by all concerned.