Knowledge of what one should have done, because it is easier to clearly reevaluate past actions or decisions than when they are being made or done. A reference to the visual acuity of normal eyesight (20/20 vision). With 20/20 hindsight, I can see now that having my brother as my legal counsel was unwise. A: "I can't believe I was ever interested in such a self-absorbed jerk." B: "Don't blame yourself. We all have 20/20 hindsight."
The ability to determine how some mistake or failure in the past could have been avoided. Often used ironically or sarcastically; sometimes stylized as "20/20 hindsight." I can see now that having my brother as my legal counsel was unwise, but I guess twenty-twenty hindsight isn't much use right now. A: "I can't believe I was ever interested in such a self-absorbed jerk." B: "Don't beat yourself about it. We all have 20/20 hindsight."
Knowledge after the fact, as in With twenty-twenty hindsight, I wouldn't have bought these tickets. This idiom uses twenty-twenty in the optometrist's sense, that is, "indicating normal vision," and hindsight in the sense of "looking back" or "reconsidering." [First half of 1900s]
twenty-twenty hindsightand 20/20 hindsight
n. an ability to figure out what one should have done after it is too late to do it. Everybody has twenty-twenty hindsight!
twenty-twenty hindsight, with
With complete after-the-fact knowledge. This expression, usually preceding a statement like “I would have done or said such-and-such,” uses the optometrist’s term for perfectly normal vision. The idea that hindsight, with its superior knowledge, is better than foresight has been stated since at least the nineteenth century. “If a man had half as much foresight as he had hindsight, he’d be a lot better off,” wrote Robert J. Burdette (Hawk-eyes, 1879). The Hollywood screenwriter Billy Wilder is quoted as having said, “Hindsight is always twenty-twenty.”