Firing is casino jargon used to refer to a player who is continuously betting large amounts of money. The term is most often used in land-based casinos due to its connection with casino chips.
As you may know, every land-based casino uses chips. When a player has a lot of chips and stacks them on top of each other, they resemble a gun barrel. For that reason, players started using terms such as “firing the first barrel” or “firing the second barrel” to refer to betting a whole stack of chips.
The phrase caught on and voila — the term firing was coined.
However, to continuously fire chips, players need to have a big bankroll, especially if they are playing poker that has numerous betting rounds, so the privilege is most often reserved for high rollers or whales.