A nasty wank is one in which the man is sneakily pleasuring himself, perhaps in a public toilet, in the car, or in a friends bedroom while they are out. The sneaky wank works well until it goes wrong, either because the man has not provided enough space to come, or is not prepared for it when it happens.
The result is usually a very wet t-shirt or other item of clothing, which will be embarrassingly obvious. The sneaky wank can also be preformed lying down, and may result in come running down the leg and becoming entwined in hair. This is almost impossible to clean up, especially in a hurry. The situation is exacerbated if a friend or relative is about to walk in or actually does.
Can lead to embarrassing situations for all involved.
Person 1: Why is there wet all over you and my bed? Were you having a nasty wank?
Person 2: Eh...no...though I would appreciate if you left the room again.
Person 1: You're sick.