It's a secret word for boys. You use it when you have to say Jerkoff
Friend: Do you want to do something later?
You: No, I got a One hand date
One Hand Date Verb. It's another way to say "Jerkoff". It is mainly used by boys/men.
Friend: Do you wanna do something today?
You: no, sorry. I've got a One hand Date. ;D
Friend: Ok