Uncle Pumblechook antagonizes Pip by treating him like he is unimportant and yet taking advantage of him. Pip considers Mr. Pumblechook “wretched company” because he does nothing but nag and harass him (ch 8, enotes etext p. 38).
Uncle Pumblechook is a bit more well-to-do than Pip’s family. He is actually Joe’s uncle, but Mrs. Joe “appropriated” him because he has money and she wants to be associated with him. Pumblechook pretends to be important, but in reality he is just an insignificant blowhard whose slight success has gone to his head.
Pip comments that he is not allowed to call Pumblechook “uncle” under the most severe penalties. Pumblechook antagonizes Pip first at Christmas dinner, when he insists he should be grateful for his upbringing and he and Wosple compare Pip to a pig.
Mr. Pumblechook further irritates Pip by arranging for him to go to Miss Havisham without asking his opinion.
[Pip fell asleep] without throwing any light on the questions why on earth I was going to play at Miss Havisham's, and what on earth I was expected to play at. (ch 7, p. 37)
When Pip returns from Miss Havisham’s house the first time, he is so annoyed by Pumblechook’s demeanor and so confused by Miss Havisham that he makes up a variety of lies about the elegant Miss Havisham. Pumblechook has only been pretending to personally know Miss Havisham, so he cannot contradict the lies.