Picture someone about to swear, who swaps the rough word at the last moment for a harmless one that starts the same way. The heat of the feeling stays; the offence is left behind. That swap seems to be what every use of darned is doing. Put it in front of a noun and you get you can say in front of anyone. Put it in front of an adjective and you get a volume . Lock it into a fixed sentence and you get the three set expressions: surprise, refusal, and I have no idea.
The source describes darned as a minced oath for damned — a softened substitute used to express contempt or exasperation. That substitution is the thread running through all the uses on this page. Note that darned is also, separately, the past form of the verb darn; the packet does not define that verb, so nothing is claimed about it here.
來源直接說明 darned 是 damned 的 minced oath，也就是把難聽的字換成無害版本的說法，用來表達不滿或受不了。這個「換字」正是整頁各種用法共同的線索。另外要分清楚：darned 也是動詞 darn 的過去式形式，那是另一條路，本頁的資料沒有定義那個動詞，所以這裡不對它的意思下任何判斷。
Wiktionary entry for darned (adjective), packet evidence wikt:darned:adj:0: "A minced oath for damned, used to express contempt, exasperation, consternation, etc."
Two branches
放在字前面的加強語氣
這一家最貼近那個「換字」的動作：情緒直接掛在下一個字上。
adjective · B2
Used in front of a noun to show that you are annoyed, fed up or impatient with someone or something. Darned stands in for a rougher swear word, so it carries the feeling without the rough word.
該死的；討厭的（客氣版）
三句固定說法：驚訝、拒絕、不知道
同一個換字的動作，這次被鎖進固定句型裡。意思屬於整句，不是 darned 這個字，所以要整組背。
adjective · B2
You say I'll be darned! when something surprises you. It is a mild, friendly way of showing that the news is not what you expected.
我的天！真沒想到！
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
darnedadj/dɑːnd/
A minced oath for damned, used to express contempt, exasperation, consternation, etc. towards someone or something.
darnedadv/dɑːnd/
Damned, extremely.
這個拼法也被記錄為下列字的變化形：
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。