To thank someone is to tell them you are grateful for what they have done or given. It’s a way of showing appreciation and good manners.
From Old English *þancian* 'to give thanks', from *þanc* 'thought, gratitude'. Originally, thanks were closely connected with the idea of thinking and remembering a kindness.
To 'thank' someone is to say, 'I’m holding your kindness in my thoughts.' Gratitude in English is literally built from the same root as thinking—it’s thoughtful memory turned into words.
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