A friendship hug is where an individual will hug another person and stealthily stick a finger up the other person's anus.
*hugs*
Guy one: DUDE! WHAT THE FUCK WAS THAT!
Guy two: What bro? It's just a friendship hug. You know, 'cause we're best bros.
Guy one: YOU JUST STUCK YOUR FINGER UP MY ASS!
Guy two: Chill bro! Friendship hugs are the best kind of hugs.