Green’s Dictionary of Slang

the final stage of inc1ipient male baldness when all the hair that rfemains forms a ‘horseshoe’ or ‘cul-de-sac’ shaped fringe at sides and back.

2019		thedermatologyclinic.com 4 Mar. 🌐 The final stage of male baldness is where the top of the hair and hairline are completely bald and only a [...] ‘cul-de-sac’ of hair remains on the back and side of the scalp where hair is generally naturally thicker.
2021		Betoota-isms 82: Cul-De-Sacs [...] 1. A balding head that has crossed over from simply a receding hairline and is now beginning to resemble a suburban dead-end street.