fusilier n.

1640s, "point of convergence," from Latin focus "hearth, fireplace" (also, figuratively, "home, family"), which is of unknown origin. Used in post-classical times for "fire" itself; taken by Kepler (1604) in a mathematical sense for "point of convergence," perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before Kepler, but it is not recorded). Introduced into English 1650s by Hobbes. Sense transfer to "center of activity or energy" is first recorded 1796.

also fuzee, type of light musket, 1660s, from pronunciation of French fusil (see fusilier). As the name of a type of match used in lighting cigars and pipes by 1832, from fusee as a variant of fuse (n.).