Swing by
Definition 1 of 1
Pro Tip 1/3Casual Intent
Implies an informal, short, and unplanned visit, rather than a scheduled or formal one. 
If I finish early, I might swing by the cafe.
Pro Tip 2/3Quick Stop
Suggests the visit will be brief, often because the person is already on the go. 
Can you swing by the store and grab some milk?
Pro Tip 3/3On the Way
Often used when the destination is convenient or along the route to the final destination. 
I'll swing by the post office since it's on my way home.