I do not know how many people other than me say, anymore,
it's as if you're saying when confronted with confusing or seemingly nonsensical speech (or on rhetorical occasions when confronted with completely sensible speech), instead of, for instance, "it's all Greek to me". (An irony.) So perhaps the intelligence which I just recently gained and which I recapitulate below is of somewhat limited interest, but I predict that those whom it interests it will interest no small amount.
bar bar bar
So! We have three remarkabilities: First, we learn from the English Wikipedia article on "Rhubarb" that
It is or was common for a crowd of extras in acting to shout the word "rhubarb" repeatedly and in an unsynchronised manner, to cause the effect of general hubbub. As a result, the word "rhubarb" sometimes is used to mean "length of superfluous text in speaking or writing", or a general term to refer to irrelevant chatter by chorus or extra actors. The American equivalent is walla. Stage actors in the United States also use word "rhubarb" repeated asynchronously in a low or murmured tone to provide background voice ambience in crowd or party scenes.
I have myself never encountered this use of the word "rhubarb" but don't wish to be truculent. It is recorded in the OED, but then so is "rhumbatron". (It doesn't mean what you'd hope.)
Second, we learn from the German Wikipedia article on "Gemeiner Rhabarber" that
Der Ausdruck „Rhabarber, Rhabarber...“ für sinnloses Geschwätz stammt daher, dass in einigen frühen Tonfilmen die Statisten angewiesen wurden, immer weiter „Rhabarber“ zu sagen, wenn z. B. für eine Marktszene eine gleichmäßige aber lebhafte Geräuschkulisse erzeugt werden sollte.
And also from the article on "Barbar" that there exists "die deutsche Redensart: Ich verstand nur „Rhabarber Rhabarber“.". (Which really only makes explicit what was already in the bit from the other article but which, in giving a complete phrase, pleases me.)
Third, in addition to learning from the first of the above-mentioned German articles that "Rhabarber" is used on the stage as a nonsense word and generally to denote nonsense, and from the English article that "rhubarb" has similar stage and (putative) general uses, we also learn from that German article that the words "rhubarb" and "Rhabarber" actually come from "barbaros"!
Der Rhabarber und der Barbar haben die gleichen Wurzeln. Sie stammen von dem griechischen Wort „barbaros“ ab, das bedeutet „fremdländisch“. Die erste Silbe seines Namens verdankt der Rhabarber dem alten Namen der Wolga. Sie hieß früher Rha und an ihrer Mündung wurde „die fremdländische Wurzel“ angebaut. Alternativ wird angegeben, dass sich das Wort vom lateinischen „Radix Barbaris“ - Wurzel der Barbaren - ableiten soll.
And presumably its use in English and German is coincidental with respect to its origin. (The OED seems to incline to the first theory mentioned, giving the "rhubarb" as derived from "rhabarbarum", thence in turn from "rha barbarum", "foreign rha. What's "rha"? Rhubarb, having been named after the Volga! So what was the native rha?) I find this delightful.
Unrelated: the German article claims that you can predict the flavor (in particular the sourness) of a rhubarb stalk from its color, while the English article seems to think the color is unrelated to its suitability for cooking (these don't outright contradict, admittedly); on the other hand, the German article doesn't mention that the leaves are poisonous, while the English article devotes a decent amount of space to that subject.