it comes from Cockney "backslang"

Ah, well, should've looked into before going by memory.

There is a French cant, Verlan, where syllables are reversed. For example, cité 'city' > té-ci > téss. Some single syllable words just ended up backwards. Polari tends to have a lot of loanwords from other languages, such as Yiddish, Romanian, etc.

Ceci n'est pas un seing.