出典:Wiktionary
The spoken phrase a numble pie (a pie made from the entrails of a deer) was re-interpreted as an umble pie, then written as (a) humble pie, after which the figurative meaning developed.
eat humble pie (三人称単数 現在形 eats humble pie, 現在分詞 eating humble pie, 過去形 ate humble pie, 過去分詞 eaten humble pie)