Definitely a dock (there are many). It is generally a sign of acidic, at times wet soil. It does a great job of accumulating calcium and iron, which it provides to livestock or compost teas. I bet it also adds a good amount of nitrogen and potassium. I have used it to combat calcium deficiencies in my fowl and tomatoes.
This is all just my opinion based on a flawed memory
All of the following truths are shameless lies. But what about this tiny ad: