The phrase “through thick and thin” is one of the oldest recorded idioms in the English language, dating back to at least the 10th century when it appeared in the Exeter Book, a collection of poetry from Anglo-Saxon England. People use this idiom when they want to describe pressing on through any obstacle, or sticking with something to the very end. One might say, for example, “they stayed with the project through thick and thin, knowing that there would be a reward at the end.”
In order to understand the origins of this idiom, it is necessary to know a little bit about the history of the English landscape. For much of England's history, the island was largely wooded, and animals grazed on what was known as “wood pasture.” The English countryside was covered in forests of varying density, interspersed with pasture, making it markedly different from the way it looks today. England was heavily deforested between the 12th and 16th centuries, when trees were harvested in large numbers to build homes and ships. Much of Western Europe, in fact, was denuded of trees during this period in history.
In the time when England was forested, people could take paths and roads to reach their destinations, but many pressed on “through thicket and thin wood,” taking cross-country journeys to travel more quickly. “Through thick and thin” is a clear shortening of “through thicket and thin wood,” and by the 1300s, most people were using this phrase thanks to Chaucer, who included the idiom in one of his works.
The idea of heavily wooded lands played a major role in English literature, well through the 20th century. Tolkien, for example, has the heroes in the Lord of the Rings going through thicket and thin wood to avoid traveling by the major roads, and to cut down significantly on travel time. As evidenced in this famous trilogy, going through thicket and thin wood could also be dangerous without an experienced guide and a good sense of direction, as heavily forested regions could become very confusing to people who were not familiar with them.
When used in a metaphorical, rather than literal, sense, the “thick” part of this idiom is meant to reflect difficulties, while the “thin” part encompasses the easier parts of life. Many people view the ability to persist through thick and thin as a positive personality trait, reflecting commitment and determination, as opposed to a tendency to give up when the going gets rough.