The word often used to refer to the monstrously huge metal bumpers found on the BMW E30 3-series from 84-87. The e30s of the year 1988 also came with d-boards however they were slightly shorter and the plastic bumpers from the 89+ e30s fit without any modification.
If you have an older e30 the plastic bumper swap is not so easy and requires some alteration to fit but nonetheless can look stunning.
D-boards can also be tucked and/or painted. The tucking method involves the owner drilling the bumper shocks and compressing them to make the bumper shorter. Painting the d-boards is also a nice touch and prevents them from making small children cry during the daytime.
"There is a family having dinner on your diving boards."
"Put your sun-glasses on, honey. There's a mean set of boards up the road."