From Latin sevērus.
1 That demands from others or imposes on itself a strict adherence to norms, principles, convictions, etc, or that does not tolerate faults, errors or deviations: severe cook, "The boss is severe but fair"
2 That is very hard or strict; that does not have mitigating factors or does not consider them: severe hangover, severe punishment, "The party was severe"
3 Exact and rigid in the observance of a law, a precept, a rule or a recipe (of birria*, for example)
*But hey, it's not just any birria, it's goat meat-Michoacán style birria. Nothing to do with the "birriamen" and those nonsense.