What is an antonym for knapsack?

There isn't a direct antonym for "knapsack" in the sense of a word that means the opposite. However, depending on the context, you could use words that imply the opposite meaning:

If you're talking about the act of carrying something:

* Leaving behind: This implies not carrying something, which is the opposite of carrying it in a knapsack.

* Discarding: This implies getting rid of something, the opposite of carrying it in a knapsack.

If you're talking about the type of bag:

* Suitcase: While not a direct opposite, a suitcase is typically larger and more formal than a knapsack.

* Purse: Again, not a direct opposite, but a purse is smaller and designed for carrying different items than a knapsack.

Ultimately, the best antonym for "knapsack" depends on the specific situation and what you're trying to communicate.