Literal translation
fool of the moon
Meaning
a fool, an idiot
Explanation
A fool, but not a harsh one: the word couioun (couillon in regional French) is in itself neither vulgar nor a severe insult, and depending on tone it can even turn affectionate, quel couillon celui-là greeting a good joke. The moon, it's believed, makes people a bit silly or fanciful: a couillon de la lune is therefore someone behaving, for a while, unreasonably or too emotionally, and it will pass along with the moon. The same logic explains the often tender sense of the word fada (bewitched by fairies), which can nonetheless turn into an insult depending on tone.