To be sloppy is to be casual, messy; it means to be careless on not do something properly. The adjective is used in football mostly with two words – defending and passing. Sloppy defending means bad defending and implies that the defender or defence were lazy and not trying or concentrating. Here’s an example in the Mirror online, and is a comment by ex-Ireland and Manchester United midfielder on an England performance: “England obviously gave a shocking goal away. It was sloppy, lazy defending.” (Mirror, July 2018).
Sloppy passing means that players are giving the ball away by passing badly, often by not hitting the ball firmly enough or not looking before passing. Here is a headline from the Athletic using this pattern: “Slow starts and sloppy passing have to be rectified if Norwich’s away form is to improve” (September 2019).
You may also hear sloppy with ‘performance’ or ‘game’ too.