A "Penny Drop" is a term used by the belligerent people of the middle class society. This word is used to define a situation that arises when a supremely STD infested ex-girlfriend proudly proclaims that she has transpired to receive an std directly from you in public for all to hear.
Dave: "Jimmy, have you been amongst the moistness of your heinous endeavours of late?"
Jimmy: "Bro, you would never believe it, I was at the ORI last night and I copped a Penny Drop.....!!!"
Dave: "Which disease this time broheim??"
Jimmy: "She claims....that it was...aids....."
Dave: "Well, you look well healthy, must be a penny drop".
Jimmy: "Dont worry bro, next time I will give her heinous fisting....no aids through the knuckles bro"