means the ability to understand a situation or event only after it has already happened, when its causes and consequences have become obvious

from A transparent compound of "hind" (meaning behind or at the back, the same word found in hindquarters and hindmost) and "sight." It began quite literally as a marksmanship term — the "hind sight" being the rear sight on a firearm, paired with the foresight at the muzzle, the two lined up to aim. From that mechanical sense it drifted, by the 19th century, into the figurative "looking backward" meaning we use today, neatly mirroring its older sibling "foresight," which had long meant looking ahead.