Popular oath can be impossible to say (9)

I believe the answer is:

ineffable

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'be impossible to say' is the definition.
I can't tell whether this definition defines the answer.

'popular oath' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'oath' becomes 'effable' (I can't explain this - if you can you should believe this answer much more).
'in'+'effable'='INEFFABLE'

'can' acts as a link.

Can you help me to learn more?

(Other definitions for ineffable that I've seen before include "Too sacred to be spoken" , "Unable to be described in words" , "impossible to describe" , "Beyond words" , "beyond description" .)

I've seen this clue in The Guardian.

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