Back order something [for a merchant] to order something that is not in stock and
then make delivery to the customer when the goods become available. (The merchant
may hold your money until the order is filled.)
In other words, an order to be filled when stock is renewed
The store didn't have the replacement part for my vacuum cleaner, so the manager back ordered it for me.
The shop had to back order some of the items on my list.
We have to back-order your sofa until the new fabric arrives.