Belgium· medium· informal
le franc est tombé
“the franc has fallen”
Meaning
To finally understand something
Explanation
Said the moment the penny drops and something finally clicks.
Example
Je lui ai expliqué trois fois, et puis d'un coup, le franc est tombé.
I explained it to him three times, and then all of a sudden, the penny dropped.
Origin
Comes from old coin-operated vending machines, where nothing happens until the coin drops and the machine releases your coffee or soup. Belgians borrowed that click of understanding for the franc, and some now joke about the euro dropping instead.
Source