c. 1600, from Latin (animal) insectum "(animal) with a notched or divided body," literally "cut into," noun use of neuter past participle of insectare "to cut into, to cut up," from in- "into" (from PIE root *en "in") + secare "to cut" (from PIE root *sek- "to cut"). The Latin word is Pliny's loan-translation of Greek entomon "insect" (see entomology), which was Aristotle's term for this class of life, in reference to their "notched" bodies.
First in English in 1601 in Holland's translation of Pliny. In zoology, in reference to a class of animals, 1753. Translations of Aristotle's term also form the usual word for "insect" in Welsh (trychfil, from trychu "cut" + mil "animal"), Serbo-Croatian (zareznik, from rezati "cut"), Russian (nasekomoe, from sekat "cut"), etc. Insectarian "one who eats insects" is attested from 1893.
Among the adjectival forms that have been tried in English (and mostly rejected by disuse) are insectile (1620s), insectic (1767), insective (1834), insectual (1849), insectine (1853), insecty (1859), insectan (1888).
It is curious that in the eyes of the Anglo-Saxon naturalists, the frog, the toad, the lizard or eft (efte), and other reptiles, were usually placed under the head of insects ; and this odd classification was preserved to rather a late period. [Thomas Wright, "Anglo-Saxon and Old English Vocabularies," 1884]