This is a bit silly and short a word, but I was totally surprised to learn it was so new! Well, the word condominium is from 1714, but it carried the meaning of "joint rule or sovereignty" and was word used in politics and international law. Until, that is, in the 1960s it got used to mean a "privately owned apartment" (people would be jointly ruling the building...), at which point that obliterated the original use. I had no clue it was so new in that sense!
Now, as promised last week--a picture of me in my Croquet getup, featuring hat and dress both from Victorian Trading Company. =)