The spoken phrase turned into a thing you can count. One thank-you is one piece of thanks: something you say, write, or give. Because it is now a thing, it can be big or small, late or quick, sent or forgotten — and it can take the shape of a sentence, a card, or a box of .
把說出口的那句 thank you 打包成一個可以數、可以送出去的東西。一個 thank-you 就是「一次道謝」：因為它已經是個物件，就有大有小、有快有慢，可以寄出去也可以忘記寄；它的形狀可能是一句話、一張卡，也可能是一盒餅乾。
From the word’s history
The origin note supplied with this word says the noun is simply thank + you compounded. That matches how it behaves: the thing is the saying, which is why a thank-you can still be just the words.
隨這個詞附上的來源說明指出，這個名詞就是 thank 跟 you 兩個字黏成一個。這和它的用法對得上：被數的那個「物件」本身就是那句話，所以一個 thank-you 可以什麼都不送，就只是話。