Seppo may have different meanings, according to who uses it and in what circumstances.
It may refer to a famous Zen Buddhist master, mentioned in plenty of koans to teach aspiring monks.
“Seppo” is also the nickname for one of the characters in Kalevala, the national epic of the Finnish people.
The nickname means “smith” and is frequently encountered in Finland, as well as other Scandinavian countries.
However, “seppo” today is mostly used by Australian people, who call Americans “seppo”-s.
The slang term, developed by Aussies derives from septic tank, since it rhymes with yank and Australians considered the citizens of the United States to be full of shit.
The slang expression dates back as far as the 1960’s.
In Australian slang, it is rather common for words to be ended with an –o suffix (i.e.: druggo for drug addict), so the long and straightforward term septic tank had been changed into “seppo”.