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").
c. 1400, also brigaunt, "lightly armed irregular foot-soldier," from Old French brigand (14c.), from Italian brigante "trooper, skirmisher, foot soldier," from brigare "to brawl, fight" (see brigade). The sense of "robber, freebooter, one who lives by pillaging" is earlier in English (late 14c.), reflecting the lack of distinction between professional mercenary armies and armed, organized criminals.
Probably then it was in the sense of skirmishers that the name of brigand was given to certain light-armed foot-soldiers, frequently mentioned by Froissart and his contemporaries. ... The passage from the sense of a light-armed soldier to that of a man pillaging on his own account, is easily understood. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]
1580s, "soldier armed with a lance," from French lancier "soldier, knight armed with a lance," from Old French lance (see lance (n.)).
"heavy-armed foot soldier of ancient Greece," 1727, from Greek hoplites "heavy-armed," as a noun, "heavy-armed soldier, man-at-arms," from hopla "arms and armor, gear for war," plural of hoplon "tool, weapon, implement." One who carries a large shield, as opposed to a peltastes, so called for his small, light shield (pelte).
"treat lightly," 1520s, from trifle (n.). Earlier "cheat, mock" (c. 1300). Related: Trifled; trifling.
1826 (adj.), "eight-footed or eight-armed;" 1835 (n.) "an eight-footed or eight-armed animal," especially an octopus, from Latinized form of Greek oktōpod-, stem of oktōpous (see octopus).