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.
The possible reason why he can not talk may be due to a mental illness.