I think we all have a good idea of what the phrase "through thick and thin" means for us today, but almost none of us has any idea of its derivation.

I subscribe to a service called "A Phrase A Week" that examines these matters, and this week that phrase came up.

However, what I found most interesting was this earliest recorded use of the phrase by Geoffrey Chaucer in Olde English, to wit:

And whan the hors was laus, he gynneth gon
Toward the fen, ther wilde mares renne,
And forth with "wehee," thurgh thikke and thurgh thenne.  

Okay, if you haven’t deciphered it, please turn the page for the translation into modern English.

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

As for the meaning of thick and thin, it basically refers to grassland (thin) and wooded land (thick).