After Fong's 1616 offering, the OED also has this from 1627:
Quote:

1627 {emem} Seaman's Gram. iv. 18 A Iury Mast, that is, when a Mast is borne by the boord, with Yards, Roofes, Trees, or what they can, spliced or fished together they make a Iury-mast.




Given that these are the written citations, the term must have been around for a while, na? Mid-to-late 16C at least?