sense of "to lower (someone) in dignity."
"Don't be so humble; you're not that great." [Golda Meir]
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 was not pronounced then) converged in the pun. Umbles, meanwhile, is M.E. numbles "offal" (with loss of n- through assimilation into preceding article), from O.Fr. nombles "loin, fillet," from L. lumulus, dim. of lumbus "loin."