Ketumbra
Oh fascinating, we're mostly wrong! https://www.languagehumanities.org/what-are-the-origins-of-the-phrase-through-thick-and-thin.htm
What are the Origins of the Phrase

The phrase "through thick and thin" dates back at least to the 10th century, and is thought to have originated from the naturally...

Language & Humanities
Mar 14 at 6:34pmWeb