70s South Jersey. “Dil-strap” was a fairly common word.
Seventies NE Ohio. “Dillweed” and “dickwad” were both relatively common insults.
I grew up on Wisconsin (born 1960). Dill weed, dill hole were common slanders in the early to late 70’s. By the early to mid 80’s those become “dick weed” or “dick hole” by the process of verbal evolution I’m guessing.
Dill is a common herb grown in Wisconsin home gardens, as home canning has always been popular here.
I have no idea if that has any thing to do with the terminology. But if you’re ever asked a question on a game show that you get right based on what you read on my post, I get a cut, dill hole!!! 
I always assumed that the “dil” prefix was related to “dildo.”
Wrong?