Arthur Heffley is the great-uncle of Greg Heffley. He doesn't say words and instead says weird sounds like "Murp" and "Rrup". He only appeared briefly in "The Ugly Truth", when the family stayed at Gammie's house for Uncle Gary's wedding. He is replaced by Clyde Heffley in the online version. Greg tends to not take what he is offering in Diary of a Wimpy Kid: The Ugly Truth at dinner
The possible reason why he can not talk may be due to a mental illness.