Definition from Wiktionary, the free dictionary
- (UK, colloquial) general greeting, mostly used by the upper classes.
- Pip pip! What's going on here?
- (UK, colloquial) used to create enthusiasm, mostly by the upper classes.
- Pip pip! Let's get out there and knock the stuffing out of 'em!
Usage notes 
Very rarely used in North America and not likely to be understood, where it is most likely to be considered humorous and is often used in a parody of British English speakers, particularly in Pip pip, cheerio! or Pip pip, old chap!.