Air gets into a liquid, tiny bubbles rise, and they pack together into a soft white layer on the surface. Three things about that layer travel into every other use of the word: it sits on top of something else, it looks light and pretty, and it is gone in a minute. Whether the layer is on hot milk, on a wave, at the corner of a mouth, or on top of a market, the picture stays the same.
From the word’s history
English took froth from Old Norse froða, from a Germanic root that also gave Old English āfrēoþan, meaning to foam. It has named bubbles on a liquid from the start, and the verb is recorded from the late 14th century.
froth 來自古諾斯語 froða（泡沫），同一個日耳曼語的根也給了古英語 āfrēoþan（起泡）。它從一開始講的就是液體上的泡泡，動詞用法在十四世紀後期就有紀錄。所以現在這些意思，都是從最原始那層泡沫延伸出去的。
Wiktionary entry for froth (packet source_ref kaikki:3f6f69a190036ae4)
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