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gray matter
grey matter [noun]
A person's intelligence
US
/ˈɡreɪ ˌmæt̬.ɚ/
UK
/ˈɡreɪ ˌmæt.ər/
Example:
It's not the sort of movie that stimulates the old grey matter much
A person's intelligence
It's not the sort of movie that stimulates the old grey matter much