Literal translation
that one has bells rattling in his head
Meaning
he's got some odd ideas, a bit crazy
Explanation
This expression is said of someone rather foolish or plainly scatterbrained, through the ridiculous, jingling image of bells. Cascavèu (bell, but also meaning baguenaude or daffodil depending on use) evokes the small, round, hollow object that tinkles at the slightest movement, traditionally associated with a fool's cap or a child's toy, far removed from serious thought. Dins sa tèsto (in his head) places the image precisely inside the skull : the person in question is said to have, in place of coherent ideas, only this hollow, pointless rattle stirring without making any sense. The expression joins a widespread tradition, found in French and other languages alike, that links sonic lightness, the bell, the cricket, the emptiness, to lightness of mind. The register is familiar and readily mocking, used to describe someone a bit mad, absent-minded, or whose ideas seem far-fetched, without necessarily implying serious madness : it targets inconsistency or momentary extravagance of behaviour.