Word Origin and History for pip-pip
slangy salutation current in Britain c.1907-1923, said by Partridge to be in imitation of bicycle horn noise.
Online Etymology Dictionary, © 2010 Douglas Harper
Examples from the Web for pip-pip
"Well, pip-pip for the present," said Freddie, moving toward the box.Jill the Reckless
P. G. (Pelham Grenville) Wodehouse