literally: “beer serious” / meaning: overly or inappropriately serious
You may have heard of something called the Flat Earth Society. This is an organization that promotes the belief that the earth is not round, but pizza-shaped.
At first, you might think this is a brilliant act of satire. Until at some point you realize: They are bierernst about this! Whenever someone approaches a matter with an inappropriate amount of seriousness, this is the perfect word choice. Like for German carnival sessions, where comedic performances are reeled off in a thoroughly ritualized format as if it were a black mass of an underground cult. So you could say: Germans take their humor bierernst!
The background of the word is the assumption that drinking wine makes you lively and cheerful, while beer makes you thoughtful and serious. Whatever the Flat Earth Society guys drank before coming up with their dipshit ideas, it must have been quite a lot!