"first coat of paint or other material given to any surface," c. 1600, verbal noun from prime (v.). Meaning "act of priming a firearm" is by 1590s; that of "gunpowder in the pan of a firearm" is from 1620s.
Entries linking to priming
prime v.
"to fill, charge, load" (a weapon, before firing), 1510s, probably from prime (adj.). General sense of "perform the first operation on, prepare (something, especially wood, etc., for painting)" is from c. 1600. To prime a pump (1769) meant to pour water down the tube, which saturated the sucking mechanism and made it draw up water more readily. Related: Primed; priming.