dyed in the wool

dyed in the wool AND dyed-in-the-wool {always before noun} (a person or a group totally dedicated to a particular cause or belief; a person who has fixed, uncompromising, deep-felt beliefs to which they are committed.) — отъявленный, закоренелый; заядлый; ~ упертый

If you describe someone as dyed-in-the-wool, you mean they have very strong opinions and will not change.

Example 1: The concert hall was packed with dyed-in-the-wool rock and roll fans last night!" Joy exclaimed. "But personally, I'm a dyed-in-the-wool supporter of folk music," she added. “As for us, we are dyed-in-the-wool lovers of opera." (idiom-magic)

Example 2: He's a dyed-in-the-wool traditionalist where cooking is concerned - he won't have any modern gadgets in the kitchen.(CIDI)

Example 2: Jack and Julia are dyed-in-the-wool ecologists who use only biodegradable products.

When you dye wool a particular colour it normally stays that colour forever. The idea is that people described as dyed-in-the-wool will stay that way forever too and will not change.

Related vocabulary:
die-hard also diehard
(stubbornly resisting change or clinging to a seemingly hopeless or outdated cause)

From die hard (медленно отмирать, быть живучим; see Old habits die hard)

Example 1: ...die-hard fans

a die-hard (a person who resists change or who holds onto an untenable position or outdated attitude; one who stubbornly resists change or tenaciously adheres to a seemingly hopeless or outdated cause)

Example 1: ...rebel die-hards who refused to surrender.