Definition

The verbal get-out-of-jail-free card you play when you've just told an inside joke to your friend group in front of confused strangers. It's code for 'you had to be there.'

Example Usage

After Kevin yelled 'Spinach McWaffles!' at the concert and everyone stared blankly, he shrugged and said 'inside.'

Source: Urban Dictionary (https://www.urbandictionary.com/define.php?term=inside&defid=2936796)