Etymologists are clear on the source of the word jalousie - it's French for jealousy - but the relationship between the emotion and the window treatments originally referred to as jalousies is not something they've speculated much about. Is it that those peering out through the original jalousie blinds were jealous of the people outside? Or is it more likely that the jealousy festered in the hearts of those outside, who could see the blinds but not the faces and lives of the people they hid? This excerpt from the October 23, 1766 entry in the Duchess of Northumberland's diary perhaps provides a clue: "Rows of Seats with Jalousies in Front that [the women] may not be seen."
1766, in the meaning defined at sense 1
French, literally, jealousy, from Old French gelus jealous