P's and Q's

The origin of p's and q's is unknown, but that doesn't mean people don't have theories about it.

(source)

I've always heard that it is because in the days of the printing press it was easy for typesetters to confuse p and q since you have to set letters backwards. Similarly, it has been suggested that it is because children learning to read may get the letters mixed up. This idea is backed up in the Oxford English Dictionary's citations of p's and q's, but there are older attestations so it's probably not the source.

Another theory is that it refers to a sailor's pea-coat and queue (a tarred pig-tail hairdo), as in, "Don't dirty your pea-coat with your pig-tail." This is unlikely only because queue is attested in 1724, which is later than p's and q's by 122 years.

It is also unlikely that it's a shortening of pleases and thank yous because that phrase is not attested until the 20th century

The only suggestion that the OED offers that can't be proven or dismissed is the idea that landlords may have confused pints and quarts of beer on a customer's account.