Green’s Dictionary of Slang

(UK juv.) a warning to a woman that her slip is showing.

1942		Berrey & Van den Bark Amer. Thes. Sl.
1993–2002		Probert Encyc. 🌐 Charlie’s dead is British slang for your underwear or petticoat is showing.
2001		OnLine Dict. of Playground Sl. 🌐 charlie’s dead n. a shout at a girl who’s [sic] petticoat was showing.
2012		(con. 1960s) E. Petulengro Caravans & Wedding Bands 86: It was well-known that if you saw a girl [...] whose slip was showing, you would simply speak the words ‘Charlie’s dead’.