A lummox is an awkward or goofy person. You might fondly refer to your klutzy best friend as a big old lummox.
Use the noun lummox when you need an informal, old fashioned word for a klutz. Though it can easily be seen as a serious insult, calling someone a lummox these days is fairly unusual and is more likely to be done in an affectionate or teasing way. Lummox comes from East Anglian slang, although its origin isn't known for sure. Some experts guess that it might stem from "dumb ox" or "lumbering."