You can picture a hazel as a low tree by a wall or a hedge, with several thin stems and, in autumn, small round nuts in green cups. Everything the word does comes off that one plant: the tree, the nut it drops, the bendy wood you cut from it, and the green-brown colour of a nut that is ripe — which is the colour people borrow for eyes.
From the word’s history
Wiktionary records hazel from Middle English hasel and Old English hæsl (hazel, shrub), from Proto-Germanic *haslaz, with cognates in Dutch hazel, German Hasel and Swedish hassel. The oldest recorded meaning is the plant itself, which is why the nut, the wood and the colour all read as later extensions of it.
來源資料記載 hazel 來自中古英文 hasel、古英文 hæsl（榛樹、灌木），再往上到原始日耳曼語 *haslaz；荷蘭文 hazel、德文 Hasel、瑞典文 hassel 同源。最早記下來的意思就是這種植物本身，所以果實、木頭和顏色都可以看成後來從它長出來的用法。
Wiktionary, "hazel"
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