look up any word, like eight ball:
 
1.
To be able to last through and handle the good times as well as the bad times. (Usually in the sense of a relationship of sorts). Thick is representative of the good times, whereas thin is representative of the bad times.
True lovers will stick together through thick and thin.

My best friend and I have been through thick and thin and are still as close as we ever were.
by spacekittens246810 December 22, 2013