The word placebo first crops up in the Bible! In medieval translations of Psalms, placebo was used as a Latin word meaning "I will please", in a sentence that iterated how "I will please the Lord in the land of the living." Later on, an application of this definition for something placating gave us the "fake substance" meaning we use today (placating, by the way, is distantly related to the word placebo, as is please). Placebo is a conjugation of placeo, which simply meant "I please". That in turn can be reduced to the infinitive placere, or "to please", which is from placare, or "to soothe" (here's that connection to placate), because soothing thins are pleasing. Placere could trace to plehk, a Proto-Indo-European reconstruction also meaning "to agree", but there are a few other proposed sources, including a word for "flat". It all depends on the ablauts. So after all that Bible stuff, the word placebo was coined in the 1200s and placebo effect in the 1900s.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.