You have to wrestle a bit with the longish word cumbersome; it's cumbersome, or kind of long and clumsy, to tumble out in a sentence. It's hard to use it gracefully.
A "cumber" is something that slows you down, and though the words aren't related, "lumber" includes big pieces of heavy wood, which might slow down the person carrying them. Moving lumber is cumbersome, because it's a heavy and awkward job. Getting your laundry down the stairs or putting a Rottweiler in the bathtub can be cumbersome too.