"Thank" seems like a pretty basic word, right? It's obviously been around for a while. Say, as long as manners. ;-) Still, there's been some interesting evolution of the word!
Interestingly, "thank" and "think" share a root--"thought, gratitude" is the meaning of the word from which it's taken, which in turn is from a word that means "think, feel." Apparently this variation came about from "thoughts" moving into "good thoughts," which leads to gratitude.
Isn't that just awesome?
Of course, it had developed an ironic sense--"You can thank her for that catastrophe"--by the 1550s, and by 1703 we were thanking people for nothing.
The phrase "thank you" (short for "I thank you") is from the 1400s, and had turned into a noun (send him a thank you) by 1792.
I hope everyone has a great week!