One picture runs under all of this: a surface covered with many small sharp points, and a hand reaching out to it without thinking. Two things always follow. The points are small and there are a lot of them, so nothing here is a deep wound — it is a sting. And the sting comes back at whoever touched, which means the word is always about contact. Look at the surface and you get the . Look at the hand and you get the skin. Move the whole picture onto a person and you get the one who snaps when you say the wrong thing; move it onto a topic and you get the question nobody wants to raise.
From the word’s history
The supplied Wiktionary entry gives the form as prickle + -y, that is, having prickles. The ending -y works the same way in dirty and hairy: it says the thing has plenty of what the first part names. So the literal reading is covered in prickles, which is exactly the first picture, and everything else is that picture used somewhere new.
資料裡的字源說明寫得很單純：prickle（刺）加上字尾 -y。-y 的作用和 dirty、hairy 裡一樣，表示「帶著很多那個東西」。所以字面上就是「滿是刺的」，正好就是第一個畫面。後面每一個意思，都可以看成把這個畫面放到新的地方去用。
Wiktionary entries for prickly supplied in this evidence packet (etymology_text: From prickle + -y; source_ref kaikki:3f6f69a190036ae4:1140448570:12888)
登入就可以看每一支怎麼連回核心畫面。