You can think of mandible as the hard biting part of a mouth — the piece that closes against another piece to hold, cut and chew. In a person or a fish it is the lower jawbone, the half that swings up and down. In a bird it is one of the two halves of the . In an ant it is one of the two small tools at the front of the mouth. The animals look nothing alike, but the word keeps pointing at the same job in each of them.
From the word’s history
Late Latin mandibula is mandō 'to chew' plus a suffix that names a tool, so the word can be read as 'the chewing tool'. That reading still fits every modern use: jawbone, beak half, insect jaw.
晚期拉丁文 mandibula 由 mandō（咀嚼）加上一個表示工具的字尾組成，字面上可以讀成「用來嚼的工具」。這條線索正好對上今天的三個用法：下顎骨、鳥喙的一片、昆蟲的大顎，都是嘴巴上負責咬的零件。
Wiktionary entry for 'mandible' — etymology_text supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:879377057)
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