means Any of various marine algae that grow in the sea, ranging from delicate green strands to vast brown kelp forests, harvested for food, thickeners, and fertilizer.

from A plain English compound, exactly what it says: 'sea' plus 'weed.' The 'weed' part is telling — 'weed' meant any wild, unwanted plant, so early English speakers filed these marine algae under 'nuisance growth from the salt water.' The slight irony is that seaweed isn't a true plant at all but algae; the name was coined long before anyone sorted out that distinction.