When your cell phone, whilst in your pocket, randomly calls a number.
Sam: (returning the pocket dial) "Hey Margie did you just call me? I answered my phone, but only heard you talking in the background."
Margie: "Oh, sorry man! I must have pocket dialed you!"
In the cell phone age, pocket dialing is when your cell phone, seemingly of its own free will, dials your friends (or worse, enemies) at inappropriate times. This could be caused by small pockets or objects in the pockets that press buttons on your phone - including the send button.
I was totally making out with this chick when I realized I had pocket dialed my girlfriend. That was the last time she heard me moan - for some reason the bitch now wants nothing to do with me.