Some plants are labeled "weeds" because of their harmful effects on people (example: poison ivy), on the animals people depend on (example: 'cowsick,' a common name for datura, needs no further explanation) or the agricultural crop plants we needs (example: dodder sucks out the plants juices and weakens them). Some plants are labeled by people as unattractive, ugly or smelly. Some plants have uncomfortable resemblances to human anatomy (not that cute turnip that looks like it has a face, but that Voodoo lily that is shaped like a ...phallus). The operative word ina ll these descriptions is people: these plants are defined by us for our purposes: but of course they are, like most things. Mabey again: "they are plants which sabotage human plans." So weed, like insect pest, is a social construction and has little to do with biology or nature, though most weeds seem to share some type of toughness in their DNA. One bottom line definition of weed? One last Mabey quote "Weeds thrive in the company of humans." They grow well whenever we disturb or destroy. Cutting a forest for a field (or a garden) creates a haven for weeds. Our penence is weeding.
1250–1300; Middle English penaunce < Anglo-French; Old French peneance < Latin paenitentia penitence