why do we have to do things for 'petes' sake? who is pete? why wont he do it himself?

Answer 1

Biblical origins. Think of St Peter. Think of the omnipresent medieval church and think of hitting your thumb with a hammer. You can't swear, else the local priests will have you up before the Bishop and the Lord alone knows what the outcome of that will be, so you exclaim, in appropriate tone of voice, "For Saint Peter's sake" and carry on erecting the shelves. This phrase was amended to "For Pete's Sake" in later, less religiously oppressive, times.

Answer 2

For Pete's sake I will do this for him.

Pete cannot come to cyberspace today as he has the flue. Pete will get back to you when he is well. By for now.