The Origins of Wonder
This form of the Side Steal, which eliminates taking the stolen card into the right palm, instead carrying the card to the top of the deck in the act of gripping the deck by its ends or squaring it, was devised by T. Nelson Downs and is described under “First Method” in his Art of Magic, 1909, p. 143.
Edward Marlo added some handling refinements and gave a name the sleight its name in The Side Steal, 1957, p. 5. He did not mention Downs.
See also Side Steal and Clip Steal.