Rhubarb is a leaf stalk that we eat. Most people have probably only eaten it in pies or tarts, and thus might consider it a type of fruit. It is after all almost always prepared in desserts with copious amounts of sugar, and often with fruits like strawberries. However, this is not the natural state of rhubarb.
It is very rare to see raw rhubarb sold, let alone for people to eat it raw. This is because raw rhubarb is not really sweet. It’s more just tart. Like, it is about the sourest thing ever. It’s actually pretty good if you just add a bunch of sugar and maybe boil it to make it soft, but it’s not something you would ever just pick and take a bite of.
This still is not the most unfruitlike—there are some very sour fruits—but its botany is what really disqualifies it. The part of the rhubarb that we eat is not the fruit. It obviously has no seeds. I think the seeds are poisonous. No, what we eat is a type of stalk called a petiole. Although a few people may consider rhubarb an honorary fruit, being botanically a fruit is still a requirement to being culinarily a fruit. Because of this, rhubarb is a vegetable despite how it is used.