Originally: the action of repeating the word ‘rhubarb’ to represent an indistinct conversation or the noise of a crowd in a theatrical production. Now also (more generally): indistinct conversation, mumbling.
1940s (in an earlier sense). Probably partly from rhubarb + -ing, and partly from rhubarb + -ing.
Originally: (of an actor) that repeats the word ‘rhubarb’ so as to represent an indistinct conversation or the noise of a crowd. Now also more generally: speaking indistinctly; mumbling.
1970s; earliest use found in The Times. From rhubarb + -ing.