You can think of second-guess as a second guess laid on top of someone's first one. Somebody has to decide, and does. Then a second person — or the same person later — puts down their own version of that decision. If the second guess lands after the result is known, it is the judgement of the : I would have done better. If it lands before, you are trying to read a decision that has not been made yet, with nothing to go on but the other person. The word is always about that extra guess, and about who made it, and when.
From the word’s history
The Wiktionary entry records second-guess as a back-formation from second guesser, originally from baseball, with first use around 1941 — the onlooker who names the better play once the ball has already been thrown.
來源資料記載這個字是從 second guesser 倒推出來的，最早出現在美國棒球場上，大約一九四一年開始使用：球已經打出去，場邊的人才說出他那個「更好的一球」。這個畫面正好解釋了今天兩個用法的重點都在時間點。
Wiktionary entry for second-guess, etymology field: A back-formation from second guesser, originally from baseball. First use appears c. 1941 (packet source_ref kaikki:3f6f69a190036ae4:917342088:9381)
登入就可以看每一支怎麼連回核心畫面。