Etymology

cockchafer (n.)

popular name of a common European beetle, the May-beetle, 1690s, from cock (n.1), in reference to its size, + chafer "beetle."

Definitions of cockchafer

cockchafer (n.)
any of various large European beetles destructive to vegetation as both larvae and adult;
Synonyms: May bug / May beetle / Melolontha melolontha
From wordnet.princeton.edu