Guardian Notes and Queries had this question once and came up with an interesting range of suggestions here

One of the quirkiest is that it arose because at one time the notes were printed at a place called Quidhampton. But I'd be inclined to see that as just an odd coincidence. I'd incline towards the Latin word quid, meaning 'what' as a likely source, helped by the fact that the same word, spelt differently, means thing in Irish.

I doubt if its got anything to do with quids of tobacco, another suggestion there.