You can think of bouncy as a picture of something that will not stay pressed down. You drop it, you land on it, you push it flat — and it comes straight back at you. Nothing else is needed: no hand, no push, no help. The spring is inside the thing itself. Every use of the word keeps that one movement, and only changes what is doing the springing: a ball off the floor, a bed under a child, hair after a night on the pillow, a person through a whole day, a beat through a song.
From the word’s history
Wiktionary records bouncy as bounce + -y, the adjective ending that turns an action into a description. So the word literally means 'having bounce in it', which is exactly the picture the modern senses share.
Wiktionary 記錄 bouncy 由 bounce（彈跳）加上形容詞字尾 -y 組成，字面就是「帶有彈跳性質的」。這條線索正好對上今天所有用法共用的那個畫面：力氣在東西自己身上。
Wiktionary entry 'bouncy' (kaikki snapshot 3f6f69a190036ae4): 'From bounce + -y.'
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