That or those belonging to us: the possessive form of we, used without a following noun, often after of.
This house is ours; ours are better; she is a friend of ours.
Used to indicate the one or ones belonging to us.
The original of the painting is ours. Their children are grown, but ours are still young.
Origin of ours
Middle English oures from oure our from Old English ūre nes-2 in Indo-European roots