What does it mean when your red eared slider turtle hisses?

When your red eared slider hisses at you, it is basically the same as when a cat hisses. They either want you to stop handling them, go away, or get out of their face. My larger and grumpier red eared slider hisses all the time, so don't take it personally if they hiss at you and think they hate you. They've just had enough. But I laugh at mine, since all you have to do sometimes is walk by him and hear a soft "ssssssss!!!" :)