From cock(“male bird”) + chafer(“beetle”). The Oxford English Dictionary speculates that the name may relate to a resemblance of antennae to coxcomb, or to the beetle’s size.[1] Compare Frenchhanneton(“cockchafer”), ultimately from Frankish*hano(“rooster”). Attested from the late seventeenth century.[1]
1793, “Premiums offered by the Society for the Encouragement of Arts, Manufactures, and Commerce, in the year MDCCLXXXVI”, in Transactions of the Society Instituted at London, for the Encouragement of Arts, Manufactures, and Commerce[1], volume 4, page 294:
To the person who shall discover to the Society an effectual method, verified by repeated and satisfactory trials, of destroying the Grub of the Cockchafer, so destructive to the roots of all sorts of Corn, Pease, Beans, and Turneps, the gold medal.
His impassioned words buzzed about my ears like cockchafers round the top of the lime-trees.
1927, Havelock Ellis, Studies in the Psychology of Sex[2], volume 2:
With regard to the playing of the female part by the weaker rats it is interesting to observe that Féré found among insects that the passive part in homosexual relations is favored by fatigue; among cockchafers it was the male just separated from the female who would take the passive part (on the rare occasions when homosexual relations occurred) with a fresh male.