My Sack is "Gigantique"

My Sack is "Gigantique" is an item purchase achievement, similar to Traveler's Tundra Mammoth.

Strategy
Bring to Haris Pilton at the World's End Inn in Shattrath, purchase the only functional item from her inventory of haute couture, and equip it.

Equipped bags don't occupy an equipment slot that other players can inspect, so the only way you can show people your impeccable taste in epic fashion accessories is to link this achievement in chat.