WAS is short for Weapon Action Switch - the control used to ‘Action’ (ready for firing) a weapon.
In-game these are the 1, 2 and 3 keys for the M230, rockets and hellfires respectively.
To WAS, or action, a weapon simply press its WAS button. To De-WAS, or De-action a weapon that is actioned, just press its WAS button again.
Warning: Even if a weapon is actioned, you cannot fire it until the aircraft is armed.
To arm/safe the aircraft, use the 4 key.