Munchkin AT: When can someone play Backpack?
 

"Backpack." You may have any number of cards in your hand.
When do you play this? is it an equipment item? or is it something you keep in your "backpack" of non-active items?

Re: Munchkin AT: When can someone play Backpack?
 

It's an Item, so it gets played whenever you'd play any other Item (any time during your turn that you're not in combat, or just after you received it). You can't turn it sideways, or that would imply you're not using it and you wouldn't get any of its abilities. Since it doesn't need Hands, isn't Headgear, Armor, or Footgear, it doesn't count as taking one of those "slots."

As I said earlier, there's no "backpack" defined by the game (aside from this one Item), so I'm not sure to what you're referring.