Trying not to contact someone that you really shouldnt contact, for example: Your ex, someone you are chatting up and dont want to appear over keen to, your friends girlfriend that you have a crush on, etc.
Alludes to the thing that teachers and parents make little kids do to make sure they dont touch things they shouldnt. Literally sitting on their hands.
Dude: 'I want to text that girl I met at the club last night...'
Friend: 'Did you text her already and she didnt reply?'
Friend: 'Sit on your hands then. You know the drill.'
'I keep wanting to call my Ex, but I know I will only embarass myself'
'You should be sitting on your hands, mate'