These ROOT-WORDS are PLAC & PLAIS from the Latin placere meaning to PLEASE. Both mean the same thing. Why are they spelled differently? PLAIS shows the French influence on the Latin word. As a result, in English, even though the forms are pronounced similarly, they are spelled differently.
1. Placebo : PLAC ebo (play chay’ bo) n.
A useless but soothing medication; something intended to soothe; gratify; conciliate