One way to hold this word: dirt that has been allowed to sit. Not a spill from this morning, but damp in the walls, rubbish nobody carries out, a place that has been left alone for months. The word always carries that second part — someone could have cleaned this and did not. That is what lets it move off walls and onto what people do, so a deal can be squalid in exactly the way a room is.
From the word’s history
Learned borrowing from Latin squalidus, from squalere meaning to be rough or dirty, and related to the noun squalor. So roughness and built-up dirt were in the word from the start.
squalid 是從拉丁文 squālidus 借進英文的，來自動詞 squālēre，意思是「又粗又髒」。同一條線上還有拉丁文 squālor，也就是今天英文的 squalor。所以「表面粗糙、積著髒東西」不是後來的引申，而是這個字本來的意思，兩個現代用法都從這裡出發。
Wiktionary via kaikki snapshot 3f6f69a190036ae4, squalid etymology 1: Learned borrowing from Latin squālidus, from squālēre (to be rough or dirty); related Latin squālor
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