Through thick and thin means staying loyal in good times and terrible ones alike. Stood by him through thick and thin, friends through thick and thin, supported them through thick and thin.
It belongs to loyalty and nothing else, and it is what people say about long friendships and long-suffering supporters. The thick and the thin were dense and open woodland, meaning every obstacle in the path.
1phrasethrough thick and thinStaying loyal in good times and terrible ones alike.
“My best friend has stood by me through thick and thin for thirty years.”
through thick and thin is a loyalty.
From thick and thin meaning dense and open woodland, hence every obstacle along a path.
From thick and thin, meaning: “thick (dense) and thin woodland; all obstacles in a path”.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms