Combine all ingredients, except chicken thighs, in a bowl and whisk until smooth. Reserve 3 tablespoons of honey mustard sauce.
Pour remaining honey mustard sauce into a large zipper bag. Add chicken thighs into bag and coat chicken in sauce. Allow to marinate for 30 minutes.
Meanwhile, preheat a grill on medium high and grease the grates.
Place chicken thighs skin side down onto grill and cook until skin is crispy, about 5 minutes. Flip thighs and continue to cook for 20-25 minutes (flipping every 5 minutes) or until internal temperature reaches 165 degrees.
During last 5 minutes of cooking, baste thighs with reserved honey mustard sauce.