From Toucan's link, above: http://www.history.uk.com/history/a-dyeing-art-jess-dyde/ http://www.history.uk.com/history/a-dyeing-art-jess-dyde/ [ctrl-f the piss]

From Toucan's link, above: http://www.history.uk.com/history/a-dyeing-art-jess-dyde/ http://www.history.uk.com/history/a-dyeing-art-jess-dyde/ [ctrl-f the piss] > Urine from London was shipped up the coast to Yorkshire, where there was a big dyeing industry, and this is the origin of the phrase "taking the piss'. > Captains were unwilling to admit that they were carrying a cargo of urine and would say that the barrels contained wine. > "No, you're taking the piss" was the usual rejoinder. You can see that accusing someone of "taking the piss" is to call them out on something embarrassing and also to accuse them of lying (about it).