Green’s Dictionary of Slang

Johnny Cotton adj.

also Dolly Cotton
[rhy. sl.]

rotten.

[UK]Everyman 26 Mar. in DSUE (1984).
[UK]J. Franklyn Dict. of Rhy. Sl.
[UK]R. Puxley Cockney Rabbit.
[UK]B. Kirkpatrick Wicked Cockney Rhy. Sl. 26: Dolly Cotton rotten.