From Longman Dictionary of Contemporary Englishthe penny (has) droppedthe penny (has) droppedBritish English informalUNDERSTAND used to say that someone has finally understood something that they had not understood before → penny
Examples from the Corpus
the penny (has) dropped• At this point the penny dropped.• Suddenly the penny dropped, and Meredith knew why he'd been prowling about the airport like an angry lion.• Then the penny dropped and he realised that the man had meant a fan- bearer.• I was about to ask Jack who it was, when the penny dropped. 