Middle English, also dorre, "buzzing or whirring insect," from Old English dora, a word of unknown origin, perhaps imitative.
Entries linking to dore
dumbledore n.
1787, a dialect word in Hampshire, Cornwall, etc. for "a bumblebee." Compare bumble-bee, also dore. The first element likely is imitative (dumble-, bumble-, humble- drumble- all seem to have been used interchangeably).