Comprehensive resource for understanding and mastering English idioms
The idiom "feel blue" means to feel sad or depressed. The figurative meaning is that the person experiencing these emotions feels weighed down or heavy like a piece of fabric with water droplets on it, causing them to appear gray and dull.
The idiom "feel blue" dates back to 1867 when it first appeared in the Oxford English Dictionary. The earliest known usage was associated with feelings of melancholy or sadness. The idiom may have originated from the fact that people who are feeling down often appear pale or gray, resembling a piece of fabric with water droplets on it. Over time, the idiom has come to be used more broadly to refer to any type of negative emotion, including anger or frustration.