Cucul la praline means stupid, idiot, ridiculous. Where does the expression come from? Not clear, but here are two stories.
Cucul is the doubling of the world cul, “butt” and means stupid or idiot. Une praline can refer to two different treats: in Belgium, it’s a chocolate candy; in France, it’s an almond covered with colored sugar. The word praline (old French praslin) comes from the Maréchal de Plessis-Praslin whose cook invented the French version of the praline in the 17th century. La praline would then have been added to the adjective cucul to emphasize on the meaning.
According to another story, the expression might come from the Praslin Island in the Seychelles, whose symbol is the coco-fesse (literally “coconut butt”, a sea coconut).
Which story do you prefer?