The Slicer of T'pire Weir Isles is a stone which, on a successful Persuasion check, can be traded to anyone for the most valuable thing they have in their possession. Taako purchased the stone from Fantasy Costco, then immediately traded it back to Garfield for the Flaming Poisoning Raging Sword Of Doom.
Origin of the Name
According to the person who submitted it as a Fantasy Costco item (Reddit comment).
It's a pun of Tupperware. You know how they sell you those things like they're gonna change your life and in the end they just end up sitting in your cabinet unused and collecting dust. My mom has so many useless Tupperware knick-knacks, but you bet they were enticing when they were being sold to her.