1
general::
Despite all obstacles or adversities, as in She promised to stand by him through thick and thin. This term alludes to penetrating a forest with both thick and sparse undergrowth. Today it is nearly always used with the idea of supporting something or someone in all circumstances, as in the example. [Late 1300s]
American Heritage Idioms
2
general::
Phrase(s): through thick and thin
Cliché through good times and bad times. • We’ve been together through thick and thin, and we won’t desert each other now. • Over the years, we went through thick and thin and enjoyed every minute of it.
McGrawhill's American Idioms And Phrasal Verbs