frivolous (adj.)youdaoicibaDictYouDict[frivolous 词源字典]
mid-15c., from Latin frivolus "silly, empty, trifling, worthless," diminutive of *frivos "broken, crumbled," from friare "break, rub away, crumble" (see friable). In law (by 1736), "so clearly insufficient as to need no argument to show its weakness." Related: Frivolously; frivolousness.[frivolous etymology, frivolous origin, 英语词源]
lightly (adv.)youdaoicibaDictYouDict
Old English leohtlice "so as not to be heavy" (of material things, but also of sleep, blows, etc.); cognate with Old Frisian lichtelik, Old High German lihtlihho, German leichtlich, Old Norse lettlega (see light (adj.1)). Meaning "frivolously, indifferently" is from early 13c.