|For the unique weapon in Fallout 4, see Spray n' Pray.|
Spray 'n Pray
Your attacks do much less damage to companions, allowing you to liberally spray an area with reckless abandon.”— In-game description, Fallout: New Vegas
Spray 'n Pray reduces the damage you do to your companions to a mere 25% of normal damage. It is primarily useful for rapid fire weapons used outside of the Vault-Tec Assisted Targeting System (low critical hit chance weapons with a large area of effect, spread or high rate of fire) and Explosives.
It is similarly effective at protecting companions from Meltdown and Heavy Handed automatic attack weapons such as the thermic lance and chainsaw, though on the latter two weapons it does not prevent them from being knocked around by Super Slam!.
Behind the scenes
Spraying and praying is a term used to describe someone firing an automatic weapon at an opponent in long bursts with little to no regard for accuracy, in hopes that the sheer amount of fire power will hit the target.