Source : Webster's Revised Unabridged Dictionary (1913)

Tap \Tap\, n. [AS. t[ae]ppa, akin to D. tap, G. zapfen, OHG.
   zapfo, Dan. tap, Sw. tapp, Icel. tappi. Cf. {Tampion},
   {Tip}.]
   1. A hole or pipe through which liquor is drawn.

   2. A plug or spile for stopping a hole pierced in a cask, or
      the like; a faucet.

   3. Liquor drawn through a tap; hence, a certain kind or
      quality of liquor; as, a liquor of the same tap. [Colloq.]

   4. A place where liquor is drawn for drinking; a taproom; a
      bar. [Colloq.]

   5. (Mech.) A tool for forming an internal screw, as in a nut,
      consisting of a hardened steel male screw grooved
      longitudinally so as to have cutting edges.

   {On tap}.
      (a) Ready to be drawn; as, ale on tap.
      (b) Broached, or furnished with a tap; as, a barrel on
          tap.

   {Plug tap} (Mech.), a screw-cutting tap with a slightly
      tapering end.

   {Tap bolt}, a bolt with a head on one end and a thread on the
      other end, to be screwed into some fixed part, instead of
      passing through the part and receiving a nut. See Illust.
      under {Bolt}.

   {Tap cinder} (Metal.), the slag of a puddling furnace.

Source : WordNet®

on tap
     adj 1: (of liquids) available by turning a tap; "ale on tap"; "hot
            and cold running water on tap" [syn: {on tap(p)}]
     2: available for immediate use; "extra personnel on tap";
        "other sports and entertainment facilities are on tap";
        "there are numerous projects on tap" [syn: {on tap(p)}]