Think of slop as a thin, half-liquid mess that will not stay where it is put. It has some weight, so it rocks; rock it enough and it climbs over the edge and spreads out flat. Everything the word does grows out of that one picture — the stuff itself sitting in a bucket, the movement of it going over the rim, and the way people throw the word at anything shapeless and not worth keeping.
From the word’s history
Wiktionary traces this branch to Middle English sloppe and Old English slyppe, and notes it is related to slip. If that line holds, the word has always been about something thin enough to slide away on its own — which is exactly the mess the modern senses describe. A separate, unrelated branch of slop meant a loose outer garment; none of the senses on this page come from it.
Wiktionary 把這一支接到中古英文 sloppe、古英文 slyppe，並註明它跟 slip（滑）同源。若照這條線索看，slop 天生就跟「稀到會自己滑掉」有關，正好對上今天那團留不住的爛糊。另外還有一條完全不同的來源，指舊時的寬外衣，本頁收的意思都不是從那裡來的。
Wiktionary, English “slop”, Etymology 2 (packet source_ref kaikki:3f6f69a190036ae4:906377685); the garment branch is Etymology 1 (source_ref …:906369084).
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