When something is sideless, it means it doesn't have sides or it's missing one or more sides.
Sideless things might be open or incomplete.
Examples:
The table is sideless, so you can sit on all sides.
She wore a sideless dress to the party.
The sideless box was easy to reach into.
The sideless cake was a little funny looking.
The sideless building looked strange and unfinished.
The sideless puzzle was missing some pieces.