E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
An immoral sect of fanatics in the 15th century; so called from Picard of Flanders, their founder, who called himself the New Adam, and tried to introduce the custom of living nude, like Adam in Paradise.
You are as hot-headed as a Picard. This is a French expression, and is tantamount to our Peppery as a Welshman.