Dick, as in penis/stupid + weed, as in, marijuana. Marijuana, which perceives you to be dopey,. Weeds,also in general, as they are in the home garden. They are a pain in the ass and annoying. Therefore weed = dopey/pain in the ass/annoying. As one word, this comes to be 'dickweed'.
John: I told you not to put the sauce in until its completely cooked.
Paul: Oh,i didnt know. Oops.
John: You've just ruined it. You fuckin dickweed!