The sound a dog makes; also the horizontal threads in weaving that go across the vertical threads called the warp.
The bark meaning comes from Old English 'wof' or imitative origins. The weaving meaning comes from Old English 'owf,' related to 'over,' because the woof threads go over and under the warp.
Woof has two completely different meanings—a dog's bark and a weaving term—but they're not related at all! It's one of English's most homonymous words, and the weaving term is actually older.
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