Green’s Dictionary of Slang

sret-sio n.

also swret-sio
[backsl.]

oysters.

[UK]Hotten Dict. of Modern Sl. etc. 130: SWRET-SIO, oysters.
[UK]Hotten Sl. Dict. [as cit. 1859].
[UK]Sl. Dict.
[UK]Farmer & Henley Sl. and Its Analogues.