H is O
H/O means 'hang on,' like when your mom is yelling at you and you need a second to process what just happened.
Your mom yells, 'H/O, I'm coming to your room in five seconds!'
Your friend texts, 'H/O, I just got yelled at by my dad.'
You're on the phone and your little brother says, 'H/O, I need a snack.'