"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."

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."  

Old English fætt "fat, fatted, plump, obese," originally a contracted past participle of fættian "to cram, stuff," from Proto-Germanic *faitida "fatted," from verb *faitjan "to fatten," from *faita- "plump, fat" (source also of Old Frisian fatt, Old Norse feitr, Dutch vet, German feist "fat"), from PIE *poid- "to abound in water, milk, fat, etc." (source also of Greek piduein "to gush forth"), from root *peie- "to be fat, swell" (source also of Sanskrit payate "swells, exuberates," pituh "juice, sap, resin;" Lithuanian pienas "milk;" Greek pion "fat; wealthy;" Latin pinguis "fat").

Meaning "abounding in comforts, prosperous" is late 14c. Teen slang meaning "attractive, up to date" (also later phat) is attested from 1951. Fat cat "privileged and rich person" is from 1928; fat chance "no chance at all" attested from 1905, perhaps ironic (the expression is found earlier in the sense "good opportunity"). Fathead is from 1842; fat-witted is from 1590s; fatso is first recorded 1943. Expression the fat is in the fire originally meant "the plan has failed" (1560s).

Spanish gordo "fat, thick," is from Latin gurdus "stupid, doltish; heavy, clumsy," which also is the source of French gourd "stiff, benumbed" (12c.), engourdir "to dull, stupefy, benumb" (13c.).

c. 1300, "determined, fixed," from Old French certain "reliable, sure, assured" (12c.), from Vulgar Latin *certanus, extended form of Latin certus "determined, resolved, fixed, settled," of things whose qualities are invariable, "established," also "placed beyond doubt, sure, true, proved; unerring, to be depended upon" (also source of Old French cert, Italian certo, Spanish cierto), originally a variant past participle of cernere "to distinguish, decide," literally "to sift, separate." This Latin verb comes from the PIE root *krei- "to sieve," thus "discriminate, distinguish," which is also the source of Greek krisis "turning point, judgment, result of a trial" (compare crisis).

The transferred sense, in reference to persons, "full of confidence in one's knowledge or judgment, made certain in reference to a matter or thing," is from mid-14c. (it also was a sense in Latin). The meaning "established as true beyond doubt" in English is from c. 1400. The meaning "indefinite, not specifically named, known but not described" is from late 14c.

Different as this seems to be from sense I, it is hardly separable from it in a large number of examples: thus, in [a certain hour], the hour was quite 'certain' or 'fixed', but it is not communicated to the reader; to him it remains, so far as his knowledge is concerned, quite indefinite; it may have been, as far as he knows, at any hour; though, as a fact, it was at a particular hour. [OED]

Lewis & Short write that Latin certus also was sometimes indefinite, "of things, the certainty of whose existence is given, but whose nature is not more definitely designated, or comes not into consideration ...."

Hence the euphemistic use, attested from mid-18c., as in woman of a certain age "an old maid;" woman of a certain description "disreputable woman;" in a certain condition "pregnant;" a certain disease "venereal disease;" of a certain weight "obese." Used with proper names from 1785, "often conveying a slight shade of disdain" [OED]. Certainer, certainest were common to c. 1750, but have fallen from proper use for some reason. Expression for certain "assuredly" is attested by early 14c.