Think of something that has gone soft because nothing firm is holding it tight any more. Press it and it gives way. Move it and it keeps wobbling for a moment after. Nothing is broken and nothing is missing — the shape is all still there. What has gone is the tension underneath that used to keep it pulled in.
From the word’s history
Wiktionary records flabby as a variant of flappy, from flap 'to hang loose', alongside dialectal flapsy and Danish flab 'the hanging lip of an animal'. The old picture of something hanging loose and flapping sits directly under the modern meaning.
Wiktionary 記載 flabby 是 flappy 的變體，來自 flap（鬆鬆地垂著、啪答晃動），並列出方言 flapsy 和丹麥文 flab（動物垂下來的嘴唇）。那個「垂著、一晃就動」的老畫面，正好就躺在今天的用法底下。
Wiktionary etymology supplied in evidence packet (kaikki:3f6f69a190036ae4:642101358:18370)
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