According to Thomas Bushnell, the primary architect of the Hurd:
It's time to explain the meaning of Hurd.
Hurd stands for Hird of Unix-Replacing Daemons. And, then, Hird stands for Hurd of Interfaces Representing Depth.
We have here, to my knowledge, the first software to be named by a pair of mutually recursive acronyms.
Quoting him further:
The name Hurd was invented by me, as an alternate spelling for the English word herd. Hird is just another alternate spelling for the same word. By the normal rules of English orthography, they all have the same pronunciations.
In other contexts:
One contributor from Norway described two other uses of Hird.
"the kings men", a name given to the men accompanying the Norwegian kings at about year 1000 and on.
This was later coined by V. Quisling when he formed a party with nationalistic traits to denote a set of helpers promoting his agenda of national and Nordic ideas.
a symbol of collaboration with the (German) enemy used in World War II.