c.1400, lefe longe (day), from leve, lief "dear" (see lief), used here as an emotional intensive + long. From late 16c. the first word was mistaken for live (v.). Ger. has cognate die liebe lange Nacht, lit. "the dear long night."
intens. of long, c.1400, leve longe; first element is lief "beloved," but over the years conformed in spelling to live as lief grew strange.