What does クーデレ Kuudere mean?

A kuudere is a character who is outwardly cool, composed and unexpressive, and inwardly attached. The type is real and well populated.

How it is built

クール is the English cool, borrowed. デレ is clipped from the doubled mimetic word for going soppy over someone, the same element in tsundere and yandere.

So the construction is an English adjective plus a Japanese fragment, assembled on the pattern of two existing words. That is a legitimate way to make a word, and it is worth noticing that the pattern is being followed rather than the language generating the term independently.

The honest position on the family

The family runs to a long taxonomy covering every temperament.

This one was coined in Japanese fandom, as a derivative of tsundere, and Japanese light novels use it in their titles.

Why the type outlives the terminology

Because the character is genuinely useful. A person whose feelings are real and undisplayed creates a specific kind of tension: the audience is asked to read evidence rather than statements, and every small gesture carries weight it would not carry from an expressive character.

That works regardless of what it is called, and it long predates the vocabulary in fiction everywhere.

What English would call it

Nothing compact. Reserved describes manner. Repressed diagnoses. The strong silent type is close, carries a gendered assumption, and belongs to a different set of stories.

So the coinage filled a real gap in fan vocabulary, which is the best case for it. It is also a Japanese coinage that English fandom borrowed, built from one borrowed part and one native one.