The word "mall" comes to us from 15th century England, after Sir Thomas Malory. When he was justly imprisoned for any number of offenses, he nevertheless continued his illicit dealings, selling stolen goods and financing various illegal activities from his cell:
He hath turned the Gaol into a very Den of Yeggs and Ill-Goers, and cares nothing to Cloake hys Iniquities. Hys Peculations and Usuriousnesse (most Contrarie to Scripture), are become as a Watch-Word even amongst ye Rogues whom he dealst wyth, and indeed each claims a Wyst that he were the Commander as it were of a MALLEORRES that is a Thieves'-Den. Which shew the unrepentante soull of thys THOMS MALLEORE styled Knyght-Presoner.The word passed into general usage as a name for a pawn shop, and soon was shortened to "mal" or "mall". It acquired its current meaning in the 19th century during the American Civil War.
Previous false etymologies: