early 14c., "heart or inmost part of anything" (especially an apple, pear, etc.), of uncertain origin, probably from Old French cor, coeur "core of fruit, heart of lettuce," literally "heart," from Latin cor "heart," from PIE root *kerd- "heart."
Meaning "a central portion cut and removed" (as from a tree, soil, etc.) is from 1640s. Meaning "internal mold of a casting, which fills the space intended to be left hollow" is from 1730. Nuclear physics sense "portion of a reactor containing the nuclear fuel and where the reactions take place" is from 1949.
"instrument for cutting the core out of an apple or other fruit," 1796, agent noun from core (v.).
It forms all or part of: accord; cardiac; cardio-; concord; core; cordial; courage; credence; credible; credit; credo; credulous; creed; discord; grant; heart; incroyable; megalocardia; miscreant; myocardium; pericarditis; pericardium; quarry (n.1) "what is hunted;" record; recreant; tachycardia.
It is the hypothetical source of/evidence for its existence is provided by: Greek kardia, Latin cor, Armenian sirt, Old Irish cride, Welsh craidd, Hittite kir, Lithuanian širdis, Russian serdce, Old English heorte, German Herz, Gothic hairto, "heart;" Breton kreiz "middle;" Old Church Slavonic sreda "middle."
"fuel residue, solid product of the carbonization of coal,"an important substance in metallurgy, 1660s, a northern England dialect word, perhaps a variant of Middle English colke "core (of an apple), heart of an onion" (c. 1400), also "charcoal" (early 15c.), a word of uncertain origin. It seems to have cognates in Old Frisian and Middle Dutch kolk "pothole," Old English -colc, in compounds, "pit, hollow," Swedish dialectal kälk "pith." Perhaps the notion is the "core" of the coal, or "what is left in the pit after a fire."
"either of two round projections of a cannon," 1620s, from French trognon "core of fruit, stump, tree trunk," from Old French troignon (14c.), probably from Latin truncus (see trunk (n.1)).