Old English leohtlice "so as not to be heavy" (of material things, but also of sleep, blows, etc.), from light (adj.1) + -ly (2). Similar formation in Old Frisian lichtelik, Old High German lihtlihho, German leichtlich, Old Norse lettlega (see light (adj.1)). Meaning "frivolously, indifferently" is from early 13c. (compare Old English leohtmodnes "frivolity").