https://wordhistories.net/2017/10/16/kings-shilling-origin
Oct 16, 2017 · The phrase to take the shilling, or to take the King’s, or Queen’s, shilling, means to sign up as a soldier, from the former practice of giving a shilling to a recruit when he enlisted. This phrase is first recorded in a note dated 27th March 1707, as published in Remarks and Collections of Thomas Hearne¹ (Oxford Historical Society – Oxford, 1886):