Someone or something that is lame, uncool, or generally disliked. They are often times from the southern part of the United States, which may or may not have a relation to the plant, "buckwheat". It can be used as a direct insult or used playfully between friends. The noun "buckweed", is not considered a swear or overly inappropriate but is not something good to be called.
"Don't touch that you stupid buckweed!"
"Dave is such a buckweed, I can't beleive he lost my phone."
"Get over here you buckweeed, you have some explaining to do."