You can picture slouch as something that has stopped holding itself up. The back rounds, the shoulders roll forward, the head drops a little, and the whole shape settles downward in a soft curve. Nothing is broken and nothing has fallen over — it is just not carrying its own weight any more. Hold that one picture and the rest follows: a person sitting or walking that way, the name for that shape, a hat brim or a plant hanging in the same soft droop, and a head let down onto a desk. Even the compliment no slouch works off it, by saying that a person is the opposite of that drooping shape.
From the word’s history
The source marks the origin as unclear. One suggested line runs through Middle English slugge and Old Norse slokr, 'a lazy fellow', with Swedish sloka 'to droop, wilt' and slak 'slack' alongside it. If that line is right, the oldest idea in the word is slackness — something loose that hangs — which is exactly the modern picture.
資料把這個字的來源標為「不確定」。一種說法是它經過中古英文 slugge、古北歐語 slókr（懶散的人），旁邊還有瑞典語 sloka（垂下、枯萎）和 slak（鬆的）。如果這條線成立，這個字最底層的意思就是「鬆」——鬆掉的東西就會垂下來，這正好就是今天的畫面。因為來源標明不確定，這裡當成一個記憶的線索，不是定論。
Wiktionary via local kaikki extract, source_ref kaikki:3f6f69a190036ae4:665330949 (etymology explicitly marked "Unclear. Perhaps…")
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