Picture soot on a finger, and then the finger dragged across a page. Nothing about the result has a clean edge: the black spreads sideways, the writing underneath goes cloudy, and you cannot say exactly where the mark stops. That softness of edge is what smudge names. Hold the picture and the verb follows at once, because the verb is simply the rubbing that makes it, either done to a thing or happening to a thing. The word has a second, smokier side too. Thick smoke has no edge either: it spreads, it drifts, it settles on whatever it reaches. English gives that cloud the same name, and the sense of a slow smoky fire, and of a bundle of herbs burned for its smoke, all live on that side.
From the word’s history
The supplied Wiktionary entry derives the noun from the verb and calls it a doublet of smutch; the verb goes back to Middle English *smogen, alongside Middle Dutch smoddich and German schmutzig, both meaning dirty. So the idea of dirt sits in the word itself, and the sooty, edgeless mark is the picture it settles on.
資料裡說這個名詞是從動詞來的，而且和 smutch 同源；動詞可以追到中古英語的 *smogen，和中古荷語 smoddich、德語 schmutzig（髒的）是同一群。也就是說「髒」本來就在這個字裡面，而它落腳的畫面，就是那種煤煙一樣、沒有邊的痕跡。
Wiktionary entries supplied in this packet: noun etymology 1 (source_ref kaikki:3f6f69a190036ae4:784276083) and verb etymology 2 (source_ref kaikki:3f6f69a190036ae4:784288716)
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