We don't have synonyms for rubarb yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are rhubarb and rubab.
(n) :
Obsolete spelling of rhubarb. [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).]
(n) :
Obsolete spelling of rhubarb. [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).]