The expression derives from umble pie, which was a pie filled with the chopped or minced parts of a beast's 'pluck' - the heart, liver, lungs or 'lights' and kidneys, especially of deer but often other meats. Umble evolved from numble, (after the French nomble) meaning 'deer's innards'.
It has occasionally been suggested [according to whom?] that 'umbles' were considered inferior food and that in medieval times the pie was often served to lower-class people, possibly following speculation in Brewer's Dictionary of Phrase and Fable but there is little evidence for this. Early references in cookbooks such as Liber Cure Cocorum present a grand dish with exotic spices.
Although "umbles" and the modern word "humble" are etymologically unrelated, each word has appeared both with and without the initial "h" after the Middle Ages until the 19th century. Since the sound "h" is often dropped in many dialects the phrase was rebracketed as "humble pie". While "umble" is now gone from the language, the phrase remains, carrying the fossilized word as an idiom.
See also 
|Look up humble pie in Wiktionary, the free dictionary.|