brigand n.

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]