Through Thick and Thin
In good times and bad times.
Examples:
George Bush stuck with the Iraq war through thick and thin.
I must follow him through thick and thin.
They remained friends through thick and thin.
Origin / Etymology
The expression is very old and originates from the times when England was still a state with predominantly forested nature and even animals grazed in wooded areas.
The idiom literally means both through dense and through sparse forest.
Meanings from the English-Bulgarian dictionary:
Thick [θik]
Thick, dense, dense.
Thin [θin]
Thin, very thin.
Sparse, thin, thinned, scanty (for hair, crops, etc.).
Teaching system;