A slob is someone who's messy or lazy. Tuck your shirt in and wash the dishes, you slob!
You may think of your sister as a slob if she never cleans up after herself, leaving crumbs all over the house and clothes all over her room. It's a derogatory term, though, so think twice before you call her a slob out loud. An earlier meaning of the word was "muddy land," from the Irish slab, or "mud." The 1860s expression "slob of a man" led to the current definition of slob.