An external lookup relationship links a child standard, custom, or external object to a parent external object.
When you create an external lookup relationship field, the values of the standard External ID field on the parent external object are matched against the values of the external lookup relationship field. For a child external object, the values of the external lookup relationship field come from the specified External Column Name.
Steps to create external lookup relationship:
1. Go to Setup –> Develop –> External Data Sources and create External Data Source.
2. Go to Setup –> Develop –> External Objects and create External Object.
3. Go to Setup –> Create –> Objects and select the object under which you want to create external lookup relationship.
Cheers!!!