A while back (here) I had a brief not on greeting—where, incidentally, I pointed out that in my own childhood we said “Hi” by using the Latin word “servus”; in Hungary that’s spelled szervusz. Today the French merci came up; words keep doing that in our morning conversations. I went searching for it in that post on “Hello” but could not find it.
My spontaneous inclination was to derive it from a longer phrase, as in the English “I’m at your mercy.” The French have a very similar phrase as in the following: “Monsieur le Président, c’est un honneur pour moi d’être ici, et je vous en remercie.” Now in that usage, the speaker is re-thanking the President because, one assumes, the invitation was itself a form of “grace” or gift, hence the Spanish gracia. It’s not a bad guess to assume that endless repetition of the “je vous en remercie” would eventually been shortened to merci. The English “I’m at your mercy” no doubt was taken from that phrase. The Wiktionary (here) derives merci from the Latin mercēs, meaning pay, reward, wages. Hence the English “I’m in your debt” would be equivalent to be at someone’s mercy. Complicated, all this. It seems that in French saying merci means receiving something—and acknowledging that situation by using the generic for “a gift”—for getting one. Suppose that we said “dollar” instead of “thanks.” Wouldn’t that give foreigners a fit in understanding English?
But the complications associated with a word like merci are incomparably simpler than to explain that little word en in that French phrase. For that task I’d have to apply to someone who understands French at a much deeper level than I understand English. Some things you simply know—and even waterboarding could not elicit a meaningful response unless you were a highly specialized linguistic scholar.