One of the many names used by the Elves for the race of Men. We're told no more about the term than the fact that it existed, so the intended meaning is not entirely clear, but it appears to simply be a reference to the fact that Men in general lacked the grace and poise that was common to the Elves. It may also suggest that the immortal Elves saw Mortal Men as impatient and given to rash acts, presumably due to their fleeting lifespans (as they would appear from an Elf's perspective).