Poor qulity marijuana, reefer, weed, pot, 420, skunk, etc. etc.
WalterBailey broke out his swag bag and wanted to add some of his reefer to Willie Joes blunt, Willie Joe exclaimed, "Yo get that schwag bag full of dill weed the hell away from my blunt bitch!" "Dat shit be some serious dill weed!" WalterBailey cried and said "I'm so uncool".
Popularized by the Beavis and Butt-head TV series, "dillweed" is a derogatory term used to describe irrational/dumb behavior and low intellectual capacity, usually resulting in some sort of consequence.
Literally, "dillweed" likens someone to pubic hair - "dill" being slang for "penis" (think dill pickle) and "weed" being slang for "hair."
The polite definition of 'dickweed'. One may want to refrain from using dickweed in mixed company at times, so they chicken out and use dillweed. Also known in some circles as dickwad. Not to be confused with dickface which is slightly more extreme.