Real Dictionary

lemming
If you say that a large group of people are acting like lemmings, you are critical of them because they all follow each other into an action without thinking about it.
countable noun: A lemming is an animal that looks like a large rat with thick fur. Lemmings live in cold northern regions and sometimes travel in very large numbers.

lemming in American English
any of various small, mostly arctic rodents (family Cricetidae) resembling mice but having short tails and fur-covered feet: some species (esp. genus Lemmus) undertake spectacular mass migrations at peaks of population growth, ultimately drowning while tr
noun: 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 in British English
noun: any of various volelike rodents of the genus Lemmus and related genera, of northern and arctic regions of Europe, Asia, and North America: family Cricetidae. The Scandinavian variety, Lemmus lemmus, migrates periodically when its population reaches a peak