deep thinker

deep cut

An obscure song by a well known musician.

deep dive

An in-depth examination or analysis of a topic.

deep down

Fundamentally; in essence; in reality

deep pockets

An ample supply of money, especially money which one is willing to spend; the possessor of such money.

deep six

To discard, get rid of, or cancel; to completely put an end to something.

deep water

A difficult or embarrassing situation.

dig deep

To make a lot of effort with all one's resources.

in at the deep end

Placed in a complex situation (especially work-related) without adequate preparation or experience.