to eat humble pie (1830) is from umble pie (1640s), pie made from umbles "edible inner parts of an animal" (especially deer), considered a low-class food. The similar sense of similar-sounding words (the "h" of humble (adj.) was not then pronounced) converged to make the pun. Umbles is Middle English numbles "offal," with loss of n- through assimilation into preceding article.
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