To rue is to feel regret or remorse for something. If that position at the deli ended up involving a reality TV show that made everyone famous, you may rue the day you turned down the job.
Rue comes from the Old English word hreowan, meaning "to make sorry," and rue can still sum up a lot of sorrow in one small syllable. Rue is a modern verb that often looks back on the past. Shakespeare made famous the phrase "rue the day," meaning you bitterly regret a moment. For example, you might rue the day you had your first coffee if you become hopelessly addicted to it. As a noun, rue can also refer to an aromatic herb.