To Pull a Mr. Fox

When you give a compliment, quickly followed by a rationalization of the circumstances, which takes away all the joy of the compliment.

Origins of how "to pull a Mr. Fox"; from The Fantastic Mr. Fox by Roald Dahl -

Mr. Fox to Ms. Fox: "By the way, you look unbelievably beautiful tonight. You're practically glowing." He then looks up and around him. "Maybe it's the lighting."