Pip-pip

A salutary British greeting dates from the very early 20th century and can mean hello or goodbye depending on the context. There are two theories about the origin. The first is that it is an abbreviation of toodle-pip. Toodle-pip and toodle-oo were common informal ways of saying goodbye at the turn of the 19th/20th century. The second theory, supported by Eric Partridge, is that pip-pip is imitative of the noise of bicycle horns that were commonplace at that time.