The limber is the two front wheels, which can be connected to rear "tail" of the cannon, the wheels of which then make a four wheeled wagon, which towed the cannon with muzzle to the rear, pointing downwards. The cannon could not be used until it had been "unlimbered", so other types of behaviour
for getting ready for action is called "unlimbering".

From the Internet:
""Who's to say?" the student asks, deflating your carefully crafted argument... "That's just your opinion," responds another, reducing argument to autobiography and making you wish you had a loaded gun... "If it's right for him, it's right," offers [a third], unlimbering the concept philosophers dread most: right-for-him. . "