The act of inadvertently calling someone on a cellular phone, when your phone is in your pocket, or purse, or some other seemingly inconspicuous place where it should not be making calls.
Your Crazy Ex: Hey, I saw that you called. What's up? Wanna hang out? Wanna get back together? I MISS YOU SO MUCH!!!
You: Eh, sorry for the confusion, but I didn't mean to call. I must have butt dialed you.
phone rings
Person 1: hello?
Persons 2, 3, 4, and 86: unintelligible drunken noise
Person 1: Is my reception bad?
Person 86: Shit, it's person 1. You just buttdialed him. Now he knows everything!
Person 1: WHAT?! HELLO?!
dial tone