To dicker is to haggle or bargain. When you buy something at a yard sale, you often have to dicker over the price.
When you dicker, you negotiate, often with some arguing or going back and forth. Your new dog walker might dicker with you over how many walks your dog needs each day, and a savvy kid will dicker with her grandmother over how many cookies she's allowed to have after lunch. The word is purely American, from the early 1800s, possibly from the old fashioned noun dicker, which was once a unit of trade that meant "a set of ten hides."