What did the word "Houyhnhnm" mean?

The Houyhnhnm are a race of intelligent horses Gulliver encounters in the 4th book. Unlike horses from 18th century England the Houynhnm rule over the Yahoos who resemble humans and basically work for them (as horses worked in 18th century England) the Houyhnhnm seemed to have an almost utopian society where lies, deceit, violence (to each other) were not recognized.