| any of various small, mouselike rodents of the genera Lemmus, Myopus, and Dicrostonyx, of far northern regions, as L. lemmus, of Norway, Sweden, etc., noted for periodic mass migrations that sometimes result in mass drownings. |

Lemming
The act of following the crowd into an investment that will inevitably head for disaster.
Investopedia Commentary
In the animal kingdom, a lemming is a rodent known for periodic mass migrations that occasionally end in drowning.
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See also: Herd Instinct