thank-ful-ness / thăngk-fəl-nəs
1. a feeling of gratitude and appreciation; gratefulness
. . . a basic law: the more you practice the art of thankfulness, the more you have to be thankful for.
Norman Vincent Peale, 1898 – 1993
A combination of the English verb thank, which is derived from the Anglo-Saxon verb thankian (thank) through the Middle English verb thankien of the same meaning, the English adjectival suffix -ful (full of), and the English noun-forming suffix -ness indicating a condition, quality, or state and derived from the Anglo-Saxon suffix -nes(s)/ -nis(s) through the Middle English suffix -nesse/-nisse both of the same meaning.
Thank you to Allen Ward for providing this etymology.