1. Marked by or given to a lack of neatness or order; untidy: a sloppy room; a sloppy roommate. 2. Showing or in the habit of using little care or attention: sloppy use of language; a sloppy researcher. 3. Informal Oversentimental; gushy: a sloppy love letter. 4. Of, resembling, or covered with slop; muddy or slushy: sloppy ground. 5. Watery and unappetizing: a sloppy stew. 6. Spotted or splashed with liquid.

It's a negative word, and literally means something that's a little bit "too liquid" when it should be more solid. For example, if you have icecream in a cone and it starts melting, it will soon become liquid and impossible to eat like a normal icecream. It can even drip and make a mess. Sloppy! This idea is applied to other phrases to mean "careless", such as sloppy habits and sloppy writing.