Green’s Dictionary of Slang

1. a street cry, often launched at passing cyclists (still a novelty in late 19C).

1909

J. Ware Passing Eng. of the Victorian Era 197/1: Pip-pip (Streets’). Hue and cry after any one, but generally a youth in striking bicycle costumery. Onomatope of the horn warning which sometimes replaces the bell of the bike.

2. goodbye! hello!; also as v.

1906		Marvel 27 Oct. 384: Good luck to you, and good-bye! Pip-pip!
1919		Wodehouse Damsel in Distress (1961) 104: ‘Toodleoo!’ ‘Good-bye.’ ‘Pip-pip!’.
1923		Wodehouse Inimitable Jeeves 84: Well, pip-pip!
1924		E.G. Murphy ‘Mrs. Flanagan’s Frock’ Dryblower’s Verses 71: Sal pulled it off and skedaddled—pip, pip!
1935		E.F. Benson Mapp and Lucia (1984) 52: Mr. Woolgar [...] did not say ‘So long’ or ‘Pip pip’.
1949		Wodehouse Mating Season 23: We had skipped the customary pip-pippings.
1953		Mad mag. Aug.–Sept. 13: Pip! Pip! The boat is in!
1959		A. Sinclair Breaking of Bumbo (1961) 44: Pip, pip, Emma.
1968		P. Larkin letter 2 Nov. in Thwaite Sel. Letters (1992) 407: Pip pip. Love to Caroleen & the Enormous Hound. Philip.
1975		(con. 1939) J. Rosenthal Evacuees Scene 29: Lots of love and keep warm. Pip, pip, pip. Your loving Mam and Dad.
1999		Indep. Rev. 4 Sept. 20: Well, in that case, pip-pip!
2000		T. Udo Vatican Bloodbath 98: Tally ho! pip pip! Wogs ahoy! Arooga arooga!!
2017		T. Spicer Good Girl Stripped Bare 59: Who in the House of Lords doesn’t indulge in sexual abuse? A dash of B&D, pip, pip, tally-ho!

3. (also pip!) a toast, cheers!

1946		J. Maclaren-Ross ‘Welsh Rabbit of Soap’ in Nine Men of Soho 10: He lifted his glass and said ‘Pip.’.
1986		S. King It (1987) 346: Richie immediately fell into the Voice of Toodles the English Butler. ‘Why, I spent it, didn’t I, guv’nor? Pip-pip, cheerio, and all that rot!’.