Player has a basic weapon, attack, or other equivalent that goes largely unused except in emergencies when ammo for primary weapon(s) runs out. Commonly too weak to use as primary weapon though and equally commonly tends to have unlimited ammo if ranged.
Alternate names: Emergency weapon, Backup weapon
The first video game about Fallback weapon was released on December 20, 1986.
Also cases of infinite supply of darts, throwing stars, arrows, bolts, or similar. Thrown weapons that return included, unless they can get stuck or otherwise fail to return (and not because of a bug).
If ammo count is shown somewhere, it usually holds the infinity symbol (∞) instead of any real count.
Weapon with these properties gained late in the game does NOT count.