feel blue

Example: “Rainy Sundays always make me feel blue.”

Origin: Blue has stood for sadness in English since at least the late 1300s — Chaucer's Complaint of Mars (c. 1385) already has "teres blewe" for a wounded heart's tears — and "blue devil," a demon of gloom and despair, is on record from the 1600s, well before "the blues" itself appears in the 1740s. Why blue specifically became the color of melancholy is genuinely unsettled: one theory points to the bluish tinge of lifeless skin and lips; another reaches back to Greek mythology's association between rain, storm, and Zeus's own sorrow, tying the color to bad weather generally; a third, more specific claim holds that ships once flew a blue flag or painted a blue band along the hull when a captain or officer died at sea, giving "feeling blue" a very literal source in mourning at sea. None of the three is confirmed over the others.