A seaweed or marine plant, possibly referring to species that grow in coastal areas or have traditionally been used as food.
From gut (possibly referring to the plant's use for digestion) plus weed (any plant), though the exact etymology is uncertain and may be dialectal.
Gutweed is one of those plant names where the original meaning has been lost to time—it might have been called 'gut-weed' because people ate it to aid digestion, or because of its slippery texture.
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