through thick and thin; through fire and water; hell and high water
Idiomatic expression meaning to go through any hardship or danger for someone or something. Often used in the pattern 〜も厭わない (won't hesitate even through fire and water).
I'd go through fire and water for him.
We've overcome every hardship, through thick and thin.
A set phrase combining 'fire' and 'water' as symbols of extreme danger and hardship, similar to the English 'through fire and water'.