What is "to prime a pump"

Prime \Prime\, v. t. [imp. & p. p. Primed; p. pr. & vb. n. Priming.] [From Prime, a.]

1. To apply priming to, as a musket or a cannon; to apply a primer to, as a metallic cartridge.

2. To lay the first color, coating, or preparation upon (a surface), as in painting; as, to prime a canvas, a wall.

3. To prepare; to make ready; to instruct beforehand; to post; to coach; as, to prime a witness; the boys are primed for mischief. [Colloq.]
   --Thackeray.

4. To trim or prune, as trees. [Obs. or Prov. Eng.]

5. (Math.) To mark with a prime mark.

   To prime a pump, to charge a pump with water, in order to put it in working condition.