Definition: Seppo is a noun originating in Australia that refers to visiting American surfers. It is no term of endearment by any means, but it isn't really a malicious slur either. Instead, it is a word that is akin to "gringo" or "haole" that are only as mean as the tone and intent of the speaker. The term seppo comes strangely from the term "septic tank" and its similarity to the word "yank" (another word for a visiting American surfer). Common knowledge makes a connection to the fact that septic tanks are full of s**t and thus, maybe, just maybe, this applies to the American surfer as well.
I can't believe it, mate. The lineup is crawling with seppos. I'm headed up coast.