plants having long green or reddish acidic leafstalks growing in basal clumps; stems (and only the stems) are edible when cooked; leaves are poisonous
Any plant of the genus Rheum, especially Rheum rhabarbarum, having large leaves and long green or reddish acidic leafstalks that are edible, in particular when cooked (although the leaves are mildly poisonous).
General background noise caused by several simultaneous indecipherable conversations, which is created in films, stage plays, etc., by actors repeating the word rhubarb; hence, such noise in other settings; rhubarb rhubarb, rhubarb rhubarb rhubarb.
Of fighter aircraft: to fire at a target opportunistically.
Of an actor in a film, stage play, etc.: to repeat the word rhubarb to create the sound of indistinct conversation; hence, to converse indistinctly, to mumble.
Of the colour of rhubarb: either brownish-yellow (the colour of rhubarb rhizomes and roots used for medicinal purposes), or pale red (often the colour of the leafstalks of common rhubarb).