Definition of slowroll
A slowroll is a tricky move that you make to tilt your opponent. A slowroll is when you take your time before calling a bet when you have the best possible hand. The goal is to have a "mind game" with your opponent. Imagine that you suspect a player of pushing all in often against you, bluffing. ?? each time, you don't have a good enough hand to call, but his aggression irritates you. To get back at him, you decide that the next time you get the nuts, you will pretend to hesitate to call. He, on the other hand, will be anxious to know whether or not his bluff will be called.
Slowroll is legal, but immoral
Obviously, there is no rule against slowroll. If you want to do it and waste your time bank on a slowroll, that's fine. However, apart from irritating your opponent, everyone loses. The game is legal, but clearly immoral. You will lose your reputation and will probably be seen as an undesirable on the table. When you have the nuts and someone pushes all in, go for a snap call instead!