flag (v.1)youdaoicibaDictYouDict
1540s, "flap about loosely," probably a later variant of Middle English flakken, flacken "to flap, flutter" (late 14c.), which probably is from Old Norse flaka "to flicker, flutter, hang losse," perhaps imitative of something flapping lazily in the wind. Sense of "go limp, droop, become languid" is first recorded 1610s. Related: Flagged; flagging.
flag (v.2)youdaoicibaDictYouDict
1875, "place a flag on or over," from flag (n.1). Meaning "designate as someone who will not be served more liquor," by 1980s, probably from use of flags to signal trains, etc., to halt, which led to a verb meaning "inform by means of signal flags" (1856, American English). Meaning "to mark so as to be easily found" is from 1934 (originally by means of paper tabs on files). Related: Flagged; flagging.