From Discworld & Terry Pratchett Wiki
Mr. Tulip's favourite syllable. This conveys a lot of very specific meaning in one small space, as in the discussion of an antique virginal:
So called because it was an instrument for –ing young ladies!
My word, was it? I thought it was just a sort of early piano!
It also has its uses in polite discourse when one does not want to have to completely spell it out.
Sacharissa Cripslock knows all about "-ing". Apparently her old teacher resorted to it a lot, but approached the phenomenon from the opposite angle; she was a nun who wanted to convey the impresssion without committing the sin of using any actual words. Indeed, towards the end of The Truth, when dealing with Ronald Carney, Sacharissa herself gleefully lets fly with a whole heavy bomber's payload of -ing's.