through thick and thin

Example: “They've stayed friends through thick and thin for forty years.”

Origin: Began as a straightforwardly literal description of pushing on through the English countryside regardless of terrain: "thicket" for dense clusters of closely growing trees or bushes, and "thin wood" for the sparser, more open stretches of forest in between. Determined, uninterrupted travel through both — the difficult, tangled parts and the easier, open parts alike — became a natural image for persistence that doesn't discriminate based on how hard or easy the going happens to be. Chaucer's writing already carries the phrase by the 1300s, and Richard Baxter's 1662 A Saint or a Brute gives an early example close to the now-standard modern wording. By the 1500s and 1600s, the phrase had shifted from its original literal terrain into a broader figure for unwavering loyalty and persistence through both hardship and ease alike, especially in describing a steadfast friendship or relationship.