Green’s Dictionary of Slang

deck n.3

[SE deck, but note deck n.2 as the convergence of seven roads means one can look in many directions; Seven Dials, near Covent Garden, was a criminal rookery n. (2) of 19C London; Monmouth Street, one of the seven streets that gave it its name, was a noted centre for second-hand clothes dealing]

Seven Dials, London, WC2; thus decker, an inhabitant of Seven Dials; on the deck, living in Seven Dials.

Show quotations
[UK]J. Ware Passing Eng. of the Victorian Era 106/2: Deck (Costers; local). The Seven Dials (W. London). ‘He’s a decker’ means he lives in the classic dials. [Ibid.] 188/1: On the deck (Costers’ local). Living in Seven Dials.