oi!

Meaning: a sound used to attract someone's attention, like "hey!"

Note: "Oi!" (with an exclamation mark) is also the name of a style of British punk rock music enjoyed mostly by skinheads. Well-known Oi! bands include Cocksparrer, The Templars, The Press, Sham 69, Blitz, Cockney Rejects, Argy Bargy, The Bruisers and The Partisans.

Origin: Possibly short for "hoi polloi", which is Greek for "the many" or "the masses". "Hoi polloi" was borrowed into English in the nineteenth century and now means "the working class" or "the lower class". Because "oi" has traditionally been used by working-class people, it's possible that it originated as an abbreviation of "hoi polloi" in order to signify a fellow member of the working classes.

Variety: This slang is typically used in British English but may be used in other varieties of English too.