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.
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)
From the picture
昆蟲與甲殼類：嘴前面那對夾子
在昆蟲身上，這對硬部件不是上下開合，而是左右夾。位置和功能還是同一個——嘴巴最前面、負責咬住食物的零件，所以名字沒有換。因為是一對，這個意思幾乎都用複數 mandibles。
noun · C1
One of the pair of hard parts at the front of an insect's mouth, used to grip, cut and chew food. Crabs, centipedes and other creatures like them have mandibles too.
大顎
noun · B2
The bone that forms the lower jaw — the part that moves up and down when a mouth opens and closes. It is used about people, animals and fish, mostly in medicine and biology.
下顎骨
noun · C1
In a bird, one of the two hard outer parts of the beak. Writers usually say which one they mean: the upper mandible or the lower mandible.
鳥喙上下兩片之一