"Slam dunk" - that sounds like something from Harry Potter. Some rather unpleasant magical creature you'd rather avoid.

If I'd been asked to guess what the word meant, I think my first guess would be, "total mess-up" - perhaps based on what might happen if you threw a large doughnut into a cup of coffee with all your strength.

Maybe that was actually what he meant. There's something to be said for using plain English in such situations.