To be able to handle the good times as well as the bad times, reflecting commitment and determination.
One unsubstantiated theory says that ‘thick’ represents the good times, and ‘thin’ the bad times. The theory suggests that the phrase may have originated in the fact that when there’s not enough food to go round, you slice meat and bread more thinly so that everyone still gets a slice, though it’s thinner.
I like that theory and it makes sense, but the explanation is actually the other way around. The origin and history of the phrase confirms that ‘thick’ means tough, and ‘thin’ means easy.
The phrase is one of the oldest recorded idioms in the English language, dating back to at least the 900s when it appeared in the Exeter Book, a collection of poetry from Anglo-Saxon England. This was when England was still a predominantly wooded country, with few roads, and the phrase originated as ‘through thicket and thin wood,’ a literal description of making progress through the ‘thick’ English countryside. At that time, people could take paths and roads to reach their destinations, but these were often “through thicket and thin wood.”
Sadly, England was heavily deforested between the 1100s and 1500s, when many, many trees were harvested to build homes and ships. Much of Western Europe, in fact, was denuded of trees during this period in history. However, there may have been benefits. According to Tolkien’s Lord of the Rings, going through thicket and thin wood could be dangerous without an experienced guide.
‘Through thick and thin,’ then, is clearly a shortening of ‘through thicket and thin wood,’ and by the 1300s, most people were using this phrase. Chaucer included the idiom in his work, The Reeve’s Tale:
“And when the horse was loose, he begins to go
Toward the fen, where wild mares run
And forth with ‘wehee,’ through thick and through thin”
The earliest citation for the contemporary wording appears to be in Richard Baxter’s religious text A Saint Or a Brute: The Certain Necessity and Excellency of Holiness, 1662:
“Men do fancy a necessity [of holiness] where there is none, yet that will carry them through thick and thin.”
I prefer to rely on Lady Luck. Sometimes she comes through. I don’t suppose she has to worry about “thick” or “thin.”