Why is Philip Pirrip nicknamed "Pip" in Great Expectations?

Pip got his nickname from the fact that he could not pronounce his name when he was young.

Pip is an orphan.  His father, his mother, and most of his siblings are dead.  He is raised by the one sister he has left.

My father's family name being Pirrip, and my Christian name Philip, my infant tongue could make of both names nothing longer or more explicit than Pip. So, I called myself Pip, and...

Pip got his nickname from the fact that he could not pronounce his name when he was young.

Pip is an orphan.  His father, his mother, and most of his siblings are dead.  He is raised by the one sister he has left.

My father's family name being Pirrip, and my Christian name Philip, my infant tongue could make of both names nothing longer or more explicit than Pip. So, I called myself Pip, and came to be called Pip. (Ch. 1)

The name keeps Pip perpetually young.  It demonstrates his sister’s lack of respect for him.  She is married to a blacksmith, Joe, who is almost childlike himself.  Joe is generally the only one kind to Pip.

Pip does get other nicknames.  After all, how long can you stick with Pip?  His friend Herbert Pocket nicknames him Handel, after a composer who wrote a song about a blacksmith.  He doesn’t like Pip’s given name any better than his nickname.

"I don't take to Philip," said he, smiling, "for it sounds like a moral boy out of the spelling-book, who was so lazy that he fell into a pond, or so fat that he couldn't see out of his eyes … (Ch. 22)

Estella does like the name Pip though, and continues to call him that.  Pip would take any name from Estella.  She is the love of his life from the time he is a small child, taken to Miss Havisham’s house to play—and train Estella in the fine art of manipulating men.

In naming his character something so diminutive, Dickens emphasizes Pip’s childishness.  He never really grows up, so he never really gets a grown-up name.  Throughout the story, Pip remains selfish and impetuous, with a big ego and very little common sense to go with it.  Money isn’t everything, and money does not make one a gentleman.