BONUS: If it's "raining cats and dogs," it's raining very hard outside. No one is sure where this phrase comes from.
額外福利：如果外面在“下貓和狗”（raining cats and dogs），說明雨下得很大。沒有人確切知道這句習語的來源。

The researchers for the study said this phrase is "an instance of rhyming slang after frogs were whipped into the air during a storm and came back down again with the rain (as testified to in historical accounts)."
研究者稱，據歷史記載，此短語是個押韻俗語，來形容“青蛙在起風暴時被卷到天上，下雨時又隨雨水摔到地面”。

Another possible origin comes from a 1592 sentence (documented by the Oxford English Dictionary) that reads: "Instead of thunderboltes shooteth nothing but dogboltes or catboltes."
另外一個出處可能是最早出現于1592年的一句話（收錄在《牛津英語詞典》中）：“這下的哪里是雷雨，分明是在下鐵釘。”。

The "dog bolts" were iron bolts to secure a door or a gate, while "cat bolts" were used to fasten together pieces of wood. In other words, they likened a heavy rainstorm to heavy metal bolts falling out of the sky. At some point, "bolt" was dropped either as a joke or to make it easier to say, causing the phrase to make no sense today.
Dog bolts是鐵螺釘，通常用來固定房門或院門，而cat bolts則通常被用來將幾塊木頭固定在一起。換句話說，這兩個詞將暴風雨比作從天上掉下來的重重的金屬釘。在某些情況下，省略bolt也許是因為玩笑，也許是因為說起來順口，所以這個習語的本意在今天并說不通。

英文來源：businessinsider
譯者：實習生孫美真
審校&編輯：丹妮