grey matter

get grey hair from

To acquire great frustration, confusion, or anxiousness from (something or someone).

give someone grey hair

To give someone great frustration, confusion, or anxiousness.

gray hat

A morally ambiguous character; an antihero.

grey area

An area intermediate between two mutually exclusive states or categories, where the border between the two is fuzzy or ill-defined.

grey cells

Brains; a person's ability to think, especially referring to intelligence.

grey power

The collective political, economic, and social influence of senior citizens, especially when they are mobilized by a common interest.

matter at hand

The current issue, which requires immediate attention.

matter of course

A natural or logical outcome; perforce.