The adverb is on the verb in this sentence, not the noun. You are saying I thank you, and how I thank you is kindly (gratefully or with grace). As opposed to thanking someone snidely or backhandedly.
Kindly could also be a shortening of “for your kindness” depending on the history of this phrase.
You are saying I thank you, and how I thank you is kindly (gratefully or with grace).
This is exactly what I’m saying - “How I thank you is kindly” i.e. “I’m being kind in the way I’m thanking you”
The earliest known use of “thank you kindly” is from a 16th-century theatrical work. It was an elaboration of the phrase “I think of you kindly” before it was shortened to “I thank you”.
“Kindly. Thank you.” Those elephant looking aliens on Mass Effect who start every sentence with the emotion they’re speaking with because they can’t speak with any inflection.
