Hold a pin and push the point once through a sheet of paper. Three things come from that one moment, and the word has kept all three. There is the hole, so small you have to look for it. Hold the paper up to a lamp and the hole becomes a tiny bright dot. And if the point went into your finger instead, there is the quick sting that is over in a second and leaves nothing behind. Hole, dot, sting: every use of pinprick is one of those three, kept at exactly that size.
From the word’s history
The source says the word is simply pin plus prick, so both parts are still visible: the tool, and the small sharp puncture it makes.
來源資料說這個字就是 pin（大頭針）加 prick（刺、刺出的小洞）組成的。兩個部分到今天都還看得見：一個是工具，一個是它刺出來的那一下。所以這個字不需要記典故，把針點在紙上的那個動作記住就夠了。
Wiktionary entries for pinprick (noun and verb), packet source_refs kaikki:3f6f69a190036ae4:1485615486:2376 and kaikki:3f6f69a190036ae4:1485617862:3376 — from pin + prick.
登入就可以看每一支怎麼連回核心畫面。