Picture someone finishing a job and back to look at what is now standing in front of them. That finished thing, still carrying the marks of the person who made it, is the picture inside handiwork. In the older use those marks are literal — the sanding, the , the paint. In the wider use the marks are simply the style of the thing: a mess, a plan, a set of rules that could only have come from one person.
From the word’s history
Wiktionary traces it to Old English handġeweorc, from hand plus ġeweorc (work). The parts already say hand-made thing, which is why the modern word still keeps that picture at its centre.
Wiktionary 把它追到古英文 handġeweorc，由 hand 加上 ġeweorc（work）組成。組成本身就是「手做出來的東西」，所以今天這個字的中心畫面幾乎沒有移動過。
Wiktionary entry for handiwork, etymology_text supplied in packet (kaikki:3f6f69a190036ae4:188016104:15183)
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