What is "upon the gad"

Gad \Gad\, n. [OE. gad, Icel. gaddr goad, sting; akin to Sw. gadd sting, Goth. gazds, G. gerte switch. See Yard a measure.]

1. The point of a spear, or an arrowhead.

2. A pointed or wedge-shaped instrument of metal, as a steel wedge used in mining, etc.

   I will go get a leaf of brass, And with a gad of steel will write these words.
   --Shak.

3. A sharp-pointed rod; a goad.

4. A spike on a gauntlet; a gadling.
   --Fairholt.

5. A wedge-shaped billet of iron or steel. [Obs.]

   Flemish steel . . . some in bars and some in gads.
   --Moxon.

6. A rod or stick, as a fishing rod, a measuring rod, or a rod used to drive cattle with. [Prov. Eng. Local, U.S.]
   --Halliwell. Bartlett.

   Upon the gad, upon the spur of the moment; hastily. [Obs.] ``All this done upon the gad!''
   --Shak.