E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The first Wednesday in Lent, so called from an ancient Roman Catholic custom of sprinkling ashes on the heads of those condemned to do penance on this day.
The ashes were those of the palms burnt on Palm Sunday. The pessimi were sprinkled with ashes, the less offending were signed on the forehead with the sign of the cross, the offlciating minister saying, Memento, homo, quia pulvis es, et in pulverem reverteris. The custom, it is said, was introduced by Gregory the Great.