A lemming is a cute mammal.
It is also the nick-name given to the worst specimen of chess-players
(you know, the one who thinks that chess problems are not interesting
and who only cares about winning games).
Ulrich Ring suggested to use this nick-name because, when shown a
quite complex Mate in 1 from Caillaud, the
lemming will shout the "solution"
Easy!!! Queen takes f7. That's mate....
after a few seconds. And only then will ask where is the problem...
Other suggested nick-names are "retro-naif", "pignorant" and "stiff-necked".
More seriously, lemmings live in northern North America (mostly Canada).
They are small, round furry mammals with tiny stubby tails. The suicide
story is emphatically false, although it is based on the fact that the
population of lemmings goes through periodic rises and falls. Just before
a population crash, there are so many lemmings that they eat almost
all available food (in limited areas) and sometimes go on mass migrations.
Since they are none too smart, they sometimes try to swim across major
rivers, thinking (presumably) that they are small tundra streams. But
there is no evidence at all of any sort of mass suicide.