Definition of penny has dropped in English English dictionary
(deyim) If the "penny has dropped" it means someone has finally realised the situation they are in after possibly being unaware of it for a long time, depending on the situation. The phrase dates back to the Victorian Era and the popular penny-slot arcades. The penny would often stick halfway down the slot and the user would then have to either wait, or give the machine a thump before the 'penny finally dropped' and they could begin playing