To lem is to do something without significant conscious thought about what it is that you are doing; to follow the crowd as lemmings are purported to do. (*) It can also mean to wander or walk, especially aimlessly or just because others are doing it. The notion that lemmings will follow each other off cliffs and to their doom is the source for both senses.
Several friends and I coined this term back in about 2001 or so. Since then, I've seen it used in blogs and even heard it in casual conversation. I can't really claim that I'm the originator of this word, since my google searches turn up some references from the 1990s, but it's still curious. It might also be worth noting that I've never seen anyone fail to recognize this term when used in context - in contrast to the term sheep, which I've had to explain before.
(*) - Yes, I am aware that lemmings don't really lem off cliffs, but the image just has too many associations with the lemmingly behaviour of the masses to ignore.