1. [n]  the discharge of a gun as signal or as a salute in military ceremonies; "a twenty gun salute"
2. [n]  a pedal that controls the throttle valve; "he stepped on the gas"
3. [n]  large but transportable armament
4. [n]  a hand-operated pump that resembles a gun; forces grease into parts of a machine
5. [n]  a weapon that discharges a missile at high velocity (especially from a metal tube or barrel)
6. [n]  a professional killer who uses a gun
7. [n]  a person who shoots a gun (as regards their ability)
8. [v]  shoot with a gun

1. \Gun\, n. [OE. gonne, gunne; of uncertain origin; cf. Ir.,
   {Gael}.) A LL. gunna, W. gum; possibly (like cannon) fr. L.
   canna reed, tube; or abbreviated fr. OF. mangonnel, E.
   mangonel, a machine for hurling stones.]
   1. A weapon which throws or propels a missile to a distance;
      any firearm or instrument for throwing projectiles by the
      explosion of gunpowder, consisting of a tube or barrel
      closed at one end, in which the projectile is placed, with
      an explosive charge behind, which is ignited by various
      means. Muskets, rifles, carbines, and fowling pieces are
      smaller guns, for hand use, and are called {small arms}.
      Larger guns are called {cannon}, {ordnance},
      {fieldpieces}, {carronades}, {howitzers}, etc. See these
      terms in the Vocabulary.
   
            As swift as a pellet out of a gunne When fire is in
            the powder runne.                     --Chaucer.
   
            The word gun was in use in England for an engine to
            cast a thing from a man long before there was any
            gunpowder found out.                  --Selden.
   
   2. (Mil.) A piece of heavy ordnance; in a restricted sense, a
      cannon.
   
   3. pl. (Naut.) Violent blasts of wind.
   
   Note: Guns are classified, according to their construction or
         manner of loading as {rifled} or {smoothbore},
         {breech-loading} or {muzzle-loading}, {cast} or
         {built-up guns}; or according to their use, as {field},
         {mountain}, {prairie}, {seacoast}, and {siege guns}.
   
   {Armstrong gun}, a wrought iron breech-loading cannon named
      after its English inventor, Sir William Armstrong.
   
   {Great gun}, a piece of heavy ordnance; hence (Fig.), a
      person superior in any way.
   
   {Gun barrel}, the barrel or tube of a gun.
   
   {Gun carriage}, the carriage on which a gun is mounted or
      moved.
   
   {Gun cotton} (Chem.), a general name for a series of
      explosive nitric ethers of cellulose, obtained by steeping
      cotton in nitric and sulphuric acids. Although there are
      formed substances containing nitric acid radicals, yet the
      results exactly resemble ordinary cotton in appearance. It
      burns without ash, with explosion if confined, but quietly
      and harmlessly if free and open, and in small quantity.
      Specifically, the lower nitrates of cellulose which are
      insoluble in ether and alcohol in distinction from the
      highest (pyroxylin) which is soluble. See {Pyroxylin}, and
      cf. {Xyloidin}. The gun cottons are used for blasting and
      somewhat in gunnery: for making celluloid when compounded
      with camphor; and the soluble variety (pyroxylin) for
      making collodion. See {Celluloid}, and {Collodion}. Gun
      cotton is frequenty but improperly called nitrocellulose.
      It is not a nitro compound, but an ethereal salt of nitric
      acid.
   
   {Gun deck}. See under {Deck}.
   
   {Gun fire}, the time at which the morning or the evening gun
      is fired.
   
   {Gun metal}, a bronze, ordinarily composed of nine parts of
      copper and one of tin, used for cannon, etc. The name is
      also given to certain strong mixtures of cast iron.
   
   {Gun port} (Naut.), an opening in a ship through which a
      cannon's muzzle is run out for firing.
   
   {Gun tackle} (Naut.), the blocks and pulleys affixed to the
      side of a ship, by which a gun carriage is run to and from
      the gun port.
   
   {Gun tackle purchase} (Naut.), a tackle composed of two
      single blocks and a fall. --Totten.
   
   {Krupp gun}, a wrought steel breech-loading cannon, named
      after its German inventor, Herr Krupp.
   
   {Machine gun}, a breech-loading gun or a group of such guns,
      mounted on a carriage or other holder, and having a
      reservoir containing cartridges which are loaded into the
      gun or guns and fired in rapid succession, sometimes in
      volleys, by machinery operated by turning a crank. Several
      hundred shots can be fired in a minute with accurate aim.
      The {Gatling gun}, {Gardner gun}, {Hotchkiss gun}, and
      {Nordenfelt gun}, named for their inventors, and the
      French {mitrailleuse}, are machine guns.
   
   {To blow great guns} (Naut.), to blow a gale. See {Gun}, n.,
      3.
   
   

2. \Gun\, v. i.
   To practice fowling or hunting small game; -- chiefly in
   participial form; as, to go gunning.
   
   
   
   

(its, from the ":GUN" command) To forcibly terminate a program or job (computer, not career). "Some idiot left a background process running soaking up half the cycles, so I gunned it."

Compare can.