Atom
late 15th century: from Old French atome, via Latin from Greek atomos ‘indivisible’, based on a- ‘not’ + temnein ‘to cut’.
wiktionary
From Middle English attome, from Middle French athome, from Latin atomus(“ smallest particle”), from Ancient Greek ἄτομος(átomos, “ indivisible”), from ἀ-(a-, “not”) + τέμνω(témnō, “I cut”).
etymonline
atom (n.)
late 15c., as a hypothetical indivisible extremely minute body, the building block of the universe, from Latin atomus (especially in Lucretius) "indivisible particle," from Greek atomos "uncut, unhewn; indivisible," from a- "not" (see a- (3)) + tomos "a cutting," from temnein "to cut" (from PIE root *tem- "to cut"). An ancient term of philosophical speculation (in Leucippus, Democritus); revived scientifically 1805 by British chemist John Dalton. In late classical and medieval use also a unit of time, 22,560 to the hour. Atom bomb is from 1945 as both a noun and a verb; compare atomic.
