c. 1300, "to rush, plunge, leap, start forth; to be set into sudden motion," from Old North French lancher, Old French lancier "to fling, hurl, throw, cast," from Late Latin lanceare "wield a lance," from Latin lancea "light spear" (see lance (n.)).
Meaning "to throw, hurl, let fly" is from mid-14c. Sense of "set (a boat) afloat" first recorded c. 1400, from notion of throwing it out on the water; generalized by 1600 to any sort of beginning. Related: Launched; launching.