The title is essentially my question. I have heard the weapon called that before, but i am curious as to both the why, and as to the origin of this term and the concept of the weapon.
A Scythe is a 2d4 damage weapon.
A brutal weapon asks you to reroll damage dice that are equal or lower in number to the weapon's brutal rating.
So a Brutal 3 Scythe automatically deals 8 damage on a hit. (As each d4 is rerolled until a 4 appears)
Make it Vorpal (L30 enchantment)
And your Brutal 3 Vorpal Scythe does infinite damage on every hit.
So every hit is an auto kill (barring any defensive funkyness) and so is a weapon worth of being weilded by Death
Alternately, it may be a reference to the weapon carried by the protagonist of the computer game, Dante's Inferno.