flag (v.) at English => English (English Etymology) Of Explained:
1545, "flap about loosely," perhaps a variant of M.E. flakken, flacken "to flap, flutter," probably from O.N. flakka "to flicker, flutter," perhaps onomatopoeic of something flapping in the wind. Sense of "go limp, droop" is first recorded 1611. Meaning "to designate as someone who will not be served more liquor" is from 1980s, probably from use of flags to signal trains, etc., to halt, which led to the verb in this sense (1856). ///