1 [always + adverb/preposition]
a long way into or below the surface of something:
Some bones were hidden deep beneath the ground.
The tunnel led deep under the mountains.
We were deep in a tropical rainforest (=far from the edge of the forest).
Tom stared deep into her eyes.
They talked deep into the night (=very late).
if you know or feel something deep down, you secretly know or feel it even though you do not admit it:
He knew, deep down, that he would have to apologise.
if someone is good, evil etc deep down, that is what they are really like even though they usually hide it:
Deep down, she is a caring person.
if things or people are two deep, three deep etc, there are two, three etc rows or layers of things or people:
People were standing four deep at the bar.
if a feeling such as hatred or anger runs deep in someone, they feel it very strongly, especially because of something that has happened in the past:
The prejudice runs deep and we need to understand the fears behind it.
to be very involved in a situation, especially so that it causes you problems