obesity n.

word-forming element meaning "toward; against; before; near; across; down," also used as an intensive, from Latin ob (prep.) "in the direction of, in front of, before; toward, to, at, upon, about; in the way of; with regard to, because of," from PIE root *epi, also *opi "near, against" (see epi-).

Proto-Indo-European root meaning "to eat," originally "to bite." 

It forms all or part of: alfalfa; anodyne; comedo; comestible; eat; edacious; edible; escarole; esculent; esurient; etch; ettin; fret (v.); frass; jotun; obese; obesity; ort; postprandial; prandial.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit admi "I eat;" Avestan ad- "to eat;" Greek edo "I eat;" Latin edere "to eat;" Lithuanian ėdu "I eat," ėdžioti "to devour, bite;" Hittite edmi "I eat," adanna "food;" Armenian utem "I eat;" Old Church Slavonic jasti "to eat," Russian jest "to eat;" Old Irish ithim "I eat;" Gothic itan, Old Swedish and Old English etan, Old High German essan "to eat."  

"exceedingly fat," 1650s, back-formation from obesity and in part from Latin obesus "fat, stout, plump," literally "that has eaten itself fat," past participle of obedere "to eat all over, devour," from ob "about; because of" (see ob-) + edere "eat" (from PIE root *ed- "to eat"). According to OED, "Rare before 19th c." Related: Obeseness. Latin obesus was translated in Old English as oferfæt "overfat." As Latin obesus also could be read as "eaten up," it also was used in a passive sense, "wasted away, lean."