The French word for "thank you" is literally "merci", which I assume is essentially short for "I'm at your mercy." The power dynamics behind gratitude are hidden in plain sight; this probably used to be taken for granted.

— Malcolm🌊Ocean (@Malcolm_Ocean) December 3, 2018

This is how so much false etymology gets started.

I don’t have time to write out the whole damn history of one word this morning, but “merci” and its usage was influenced by Christianity around the 12th Century or so after originally coming into Old French by way of Latin, where its cognates “mercēd-” etc. originally meant something like “wages” and “price” and other commerce-related ideas.

Due to some Christian doctrine arguments, the word eventually came to mean something like “blessing” or “reward” (in a more metaphorical sense) and this is when it also became used as an expression meaning “thanks” in general French usage. In other words, the “merci” you were offering your benefactor as thanks was the reward or blessing (the payment) for the good deed done for you. It had no direct sense of being at someone’s mercy.

Please be aware that I am simplifying and eliding a whole lot of history here, but I hate false etymology and there is so very much of it.