Unwashed can be pictured as the moment before the water: a cup, a hand, a shirt that the tap has not touched yet. Un- simply takes the washing away. What is left is usually dirt, and always the feeling of something still waiting to be dealt with.
The word is un- plus washed, recorded in Middle English as unwasched, so the picture has never been anything other than the washing that did not happen.
這個字就是 un- 加 washed，中古英語寫作 unwasched。所以它的畫面從頭到尾都只有一件事：那個沒發生的清洗。
Wiktionary entry supplied in this packet (kaikki:3f6f69a190036ae4:1021493176): From Middle English unwasched, equivalent to un- + washed.
Two branches
東西還沒洗
這一支就是畫面本身，用在杯子、手、衣服這類具體的東西上。
adjective · B2
Not washed yet. Most of the time this means something is still dirty and waiting to be cleaned, but it can also simply mark that nothing has been rinsed yet, as with unwashed rice or unwashed fruit.
沒洗的
拿來說人的那一群
把「沒洗過」從東西移到人身上，用身體的髒去說一群人的出身和教育。這是概念上的比喻，不是另一條字源。
adjective · C1
A name for ordinary people taken as one large crowd — poor, not much educated, and treated as beneath the speaker. It is nearly always said as a joke, often by someone making fun of that snobbish view.
市井小民（戲謔貶稱）