http://www.urbandictionary.com/define.php?term=Betty See the link. It's youth slang and not in mainstream usage in the UK. It may be Australian slang. I had never heard of it before (perhaps because I am not young any more!).

Whenever I hear this term I always think of Betty from the Archie comic books that I used to see when I was very young. I don't think that this is the source of it, although it could be, but the association works fine nevertheless.

It means she's pretty. I think the phrase was made popular by the film 'Clueless', where they use it a lot. Maybe it was popular in the US in the 90s, but I don't think anyone really says it any more.

Thank you. I found this word in a book for teens by Rainbow Rowell.