The oldest known source, but questionable explanation for the expression X dates to the 13th century in one of the earliest English statutes, instituted during the reign of Henry III (1216–1272), called the Assize of Bread and Ale. ___ who were found to have shortchanged customers could be subject to severe punishment. To guard against the punishment of losing a hand to an axe,they would give thirteen for the price of twelve, to be certain of not being known as a cheat. Specifically, this practice was insurance against "short measure", on the basis that one of the thirteen could be lost, eaten, burnt, or ruined in some way. The practice can be seen in the guild codes of the Worshipful Company of ___ in London.
give me X.