Picture yourself holding something and trying to get the meaning out of it, and failing. The paper is right there in your hand; the meaning stays shut. That is the one picture under every use of unreadable, and the uses differ only in what is holding you out. Sometimes it is the shapes on the page. Sometimes the shapes are clear but the writing wears you down before the end. Sometimes what you are trying to read is a person's face. And sometimes it is not you reading at all, but a machine that cannot get at what is stored. Same closed door each time; different lock.
From the word’s history
Wiktionary builds the word as un- + readable, and readable as read + -able. So it literally says 'cannot be read', and each modern use is just a different answer to the question: read by whom, and stopped by what?
依 Wiktionary 的說明，這個字是 un- 加 readable，而 readable 又是 read 加 -able。字面上就是「讀不了的」。今天的每一個用法，都只是在回答同一個問題的不同版本：是誰在讀，又是什麼把他擋住？
Wiktionary entry for unreadable, etymology_text in this packet (kaikki:3f6f69a190036ae4:692450312:18929)
登入就可以看每一支怎麼連回核心畫面。