One way to hold this word: you cut off a head and two grow back. That single rule is the whole word. It explains why the creature in the story is famous, why a monster type in games copies it, why a tiny pond animal that buds new bodies got the name, and why a problem that returns every time you solve it is called this. If a sentence does not have something coming back, this is probably not the word.
From the word’s history
The source traces the name to Latin Hydra, from Ancient Greek húdra, from a root meaning aquatic, and notes it is a doublet of otter. So the oldest layer is simply a water creature; the many heads come from the story told about it.
來源說這個名字來自拉丁文 Hydra，再往前是古希臘文 ὕδρα（húdra），更早的字根意思是「跟水有關的」，而且跟 otter（水獺）同源。所以最底層只是「住在水裡的東西」。那些頭，是後來故事加上去的。
Wiktionary (kaikki) entries for Hydra and hydra, supplied in packet: 「From Latin Hydra... from Ancient Greek ὕδρα (húdra), from Proto-Indo-European *údreh₂ (aquatic). Doublet of otter.」
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