Picture one small object: a low, round container with a flat lid, the size of your palm. It is wider than it is tall, it has no handle and no shape of its own beyond that squat circle, and you can set it down anywhere. Hold that picture. One meaning is the object itself, with pills inside. The other two can be thought of as the same squat circle at other sizes — poured in concrete and set on the ground, or turned over and placed on a head.
From the word’s history
Wiktionary records the word simply as pill + box: the box that pills go in. The other two meanings are linked here by shape only, as a way to remember them, not as a claim about how they came about.
Wiktionary 記的來源很單純：pill（藥丸）+ box（盒子），就是裝藥丸的盒子。至於碉堡和帽子，這裡只是把「形狀很像」當成幫助記憶的線索，不是在說它們是從藥盒一路演變出來的歷史。
Wiktionary, "pillbox"
登入就可以看每一支怎麼連回核心畫面。