There are three possible defenitions for this phrase;
1) The IPv6 alphabet. (IPv6 being an internet protocal, each IPv6 enabled PC is assigned an individual IPv6 addresse to connect to the internet.)
2) Demented cow sitting in a field, said to be dead beef as it will soon become a burger.
3) A cow found to be dead in a field.
A) Oh look, a dead cow (dead:beef), thet's eat it!
B) Oh look, my Linux box has been assigned an odd IPv6 addresse.
C) ..... has an IPv6 reverse DNS entry for their IRC bouncer.