Picture a surface with points standing out of it: a , a chestnut case, the top of a fence. That is the whole word. Spike is a long pointed nail, and -y means 'having that quality', so spiky is 'covered in points'. Hair can be made into points in the same way. A person, a song or a piece of writing can feel like points too — instead of a smooth surface, you keep meeting small sharp ends.
From the word’s history
The source traces this spiky to spike 'a piece of pointed metal' plus the adjective suffix -y, with spike itself going back through Middle English for a large nail. So the modern word can be read straight off its parts: having spikes.
來源把這個 spiky 拆成 spike（尖銳的金屬物、長釘）加上形容詞字尾 -y；spike 本身則可追到中古英語，指大鐵釘之類的東西。所以這個字可以直接從零件讀出來：帶著尖刺的。
Wiktionary entry for 'spiky', etymology 2 — etymology_text supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:1385001808)
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