(transitive, idiomatic) Use a flag or similar kind of signal to get someone's attention.

(Australia) September 3, for the first flying of the Australian flag in 1901, Australian National Flag Day.

(nautical) A black and yellow flag (signal flag LIMA) formerly hoisted by a ship to show that it has contagious or infectious disease aboard, or by a ship that has been quarantined. This use was listed in Brown’s 1916 code of signals, but this use is no longer listed in contemporary editions of the International Code of Signals.

One who carries a flag, especially at a ceremony. / One who openly promotes an idea or value and becomes symbolic for it.

(figuratively) A cue, warning, or alert; a sign or signal that something is wrong. / (idiomatic, US) red rag

A perennial wetland plant, Acorus calamus, with an aromatic medicinal root, formerly used to strew floors.

The act of burning a flag, especially the flag of a nation as an act of protest at that nation's activities.

(motor racing) A black-and-white chequered flag which signifies the end of a race. / (motor racing) A victory in a race.

present participle of flag down

(figuratively) Registration of a ship in a country chosen for its low taxes, permissive regulations, and so on.