'Relaxing'? Terrifying! (8)

I believe the answer is:

chilling

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

''relaxing'?' is the first definition.
(I've seen this before)

'terrifying' is the second definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?

Can you help me to learn more?

(Other definitions for chilling that I've seen before include "Horrifying" , "Relaxing" , "terrifying!" .)

I've seen this clue in The Everyman puzzle in The Observer.

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