Today’s expression is être à côté de la plaque. It literally translates “to be next to the target” or “to be way off the mark.” This informal expression is used to describe someone that has no clue or is way off the mark. The French word plaque has all kinds of meanings, but generally, it refers to some kind of sheet, plate, slab, or chunk of material (ice, metal, food, money, etc.). It can also mean a target (literal or figurative). In the expression être à côté de la plaque, it's referring to some target of comprehension that you're aiming for and missing.