The quality or state of being grateful; thankfulness and appreciation.
From 'grateful' (from Old English 'grace') plus the noun-forming suffix '-ness.' The word combines the concept of grace with the abstraction of a feeling or state.
The word 'gratefulness' is interesting because it adds '-ness' to 'grateful,' but we also say 'gratitude'—they mean almost the same thing but come from completely different sources: one from Germanic roots, one from Latin, showing English's dual heritage.
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