This English expression is fairly commonly used in both spoken and written language.
The expression is used to mean through both good and bad times. Generally it is given as comforting words of support from one person to another through the phrase “I will be there with you through thick and thin.”
This expression has been being used for centuries in the English language and is one of the oldest idioms which is still in use. It is said to date back to a time when England was mainly woodland and thus the phrase was originally “through thicket and thin wood.” This expression was used in a literal sense and the first record can be found in Geoffrey Chaucer’s The Reeve’s Tale written in Old English which reads “toward the fen, ther wilde mares renne, And forth with “wehee,” thurgh thikke and thurgh thenne.”
The first usage of the modern version can be found in Richard Baxter’s A Saint Or a Brute: The Certain Necessity and Excellency of Holiness, from the year 1662. It reads “men do fancy a necessity [of holiness] where there is none, yet that will carry them through thick and thin.”
An example of the usage of this expression can be found in J.R.R. Tolkien’s The Fellowship of the Ring which reads “You can trust us to stick to you through thick and thin – to the bitter end. And you can trust us to keep any secret of yours – closer than you yourself keep it.”