A Jacobus is an English gold coin of the reign of James I, worth 25 shillings.
Isaac Newton refers to the coin in a letter to John Locke:
The Jacobus piece coin'd for 20 shillings is the hide41 : part of a pound Troy, and a Carolus 20s piece is of the same weight. But a broad Jacobus (as I find by weighing some of them) is the 38th part of a pound Troy.