"Charlie's dead" means that someone's slip is showing.
(Slip means petticoat and of course this is from a time gone by, when ladies wore petticoats. "Charlie's dead" was said when a ladies' petticoat was showing below her dress or skirt, along with the equally bizarre "It's snowing down south", which probably started because ladies' petticoats were usually white. There are a couple of different explanations for Charlie's dead though.
The first explanation is that the Jacobites wore white ribbons to identify themselves to each other as supporters of "Bonnie Prince Charlie. And after he died you could point to someone who had a piece of white in their clothing and say "Charlie's dead".
The other explanation that I found is that it may stem from Charles I, where apparently at his execution the women in the front row dipped their petticoats in his blood.
Of course, like most idioms, nobody really knows for sure.)