The origin of the word ratatouille is a matter of some dispute. Some dictionaries suggest that it is a blend of the French verbs ratouiller , to shake, and touiller , to stir--though, as far as I know, the dish is not shaken in its preparation. At least one source proposes the French adjective ratatine, shrunken or shriveled, as the derivation, presumably because of the appearance of the vegetables in an overcooked version of the dish.
On the other hand, there is a Genoese word rattatuia (which may be related the Italian carabattole , baubles), commonly defined as remains or residue (with a figurative meaning of something bad, or of little value), which might also be the source of the word.
Just to confuse matters, in Menton, a dish very similar to ratatouille, but with potatoes added, is known as giambalaia . (The Louisiana Creole concoction known as jambalaya borrows its name from a similar Provencal word.) And, according to Jean and Daniele Lorenzi, in their book "Cuisine monegasque," the traditional name for ratatouille in Monaco is brandayun .
This is doubly confusing. Not only does the word suggest the brandade of stockfishknown in Liguria as brandacujun ; it is almost identical to the Monagasque word for Jerusalem artichoke--which is branduyun or brandugliun . The Lorenzis report that they've found one local cook who actually puts Jerusalem artichokes in her ratatouille--but they suspect she only does so because the similarity of names leads her to think she's supposed to.