early 13c., lustliche, "willingly, eagerly, readily;" see lusty + -ly (2). Meaning "with pleasure, voluptuously" is c.1300; meaning "vigorously, energetically" is c.1400.
early 13c., "joyful, merry," from lust + -y (2). It largely has escaped the Christianization and denigration of its root word. The sense of "full of healthy vigor" is from late 14c.; that of "full of desire" is attested from c.1400. Related: Lustily; lustiness.