What does "all" mean?
Great question! The word "all" in this situation is used to mean "completely" or "extremely". Unlike "your hand's sweaty", saying "your hand's all sweaty" emphasizes that there is a lot of sweat on his hand, because it is completely sweaty. You can use "all" in similar contexts to have the same meaning. Ex: Don't walk inside, your shoes are all muddy. Ex: I can't see anything, my glasses are all dirty.