An Etymological Dictionary of the German Language/Brunft
Brunft, f., ‘rutting-time,’ from MidHG. brunft, f., ‘fire, heat, rutting season of deer, cry.’ The MidHG. brunft is of dual origin; in the sense of ‘heat’ it belongs to brennen, Brand. Brunft, ‘the rutting season of deer,’ was rightly connected, as early as Lessing, with brummen, since it “indicates the impulse of certain animals to copulation, that is to say, of those that roar or bellow in the act; ignorance and negligence have transformed this word into Brunft” (Lessing).