Old English flicorian "to flutter, flap quickly and lightly," originally of birds. Onomatopoeic and suggestive of quick motion. Sense of "shine with a wavering light" is c.1600, but not common till 19c. Related: Flickered; flickering.
1849, "wavering, unsteady light or flame;" 1857 as "a flickering," from flicker (v.).