You can think of the dodo as the bird everyone knows is gone. It was heavy, it could not fly, it lived on one island, and then there were none left — only bones and a few old drawings. That picture is unusually complete, so English keeps borrowing it. When a plan or a rule or a machine is finished for good, it is dead as a dodo. When a person or a company stays in the old world while everyone else moves, they are a dodo. And because the bird was slow on its feet and unafraid of people, an American speaker can call someone a dodo for being slow to catch on.
From the word’s history
Wiktionary records the origin as uncertain: perhaps from obsolete Portuguese doudo (fool, simpleton) or from Dutch dodaars, first attested in the 17th century. If the Portuguese story is right, the idea of a fool was in the name before the bird ever became a byword for being gone.
Wiktionary 記載 dodo 的來源不確定：一說來自已廢用的葡萄牙文 doudo（傻瓜、笨），一說來自荷蘭文 dodaars，十七世紀開始有紀錄。如果是前者，「笨」這個意思在這個名字裡本來就有，比後來「絕種」的比喻更早；但這條線索本身沒有定論，只能當作參考。
Wiktionary, "dodo" (etymology 1)
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