In fact, that's how the sauce was created: sometime in the 1980s, a restaurant employee accidentally mixed barbecue sauce with a homemade honey mustard recipe that the owner-operator of that restaurant, Hugh Fleming, had come up with as a dipping sauce for chicken nuggets. The combination of the two sauces became so popular that they even bottled it for customers.
So if you're at home or your local Chick-fil-A runs out, you can get a reasonable substitute for Chick-fil-A Sauce by mixing some barbecue sauce and honey mustard.
Photo via Chick-fil-A.