Idiom (D)

Dark Horse:
One who was previously unknown and is now prominent.

Dead Ringer:
100% identical. A duplicate.

Devil's Advocate:
Someone who takes a position for the sake of argument without believing in that particular side of the arguement. It can also mean one who presents a counter argument for a position they do believe in, to another debater.

Dog Days of Summer:
The hottest days of the summer season.

Don't count your chickens before they hatch:
Don't rely on it until your sure of it.

Don't Look A Gift Horse In The Mouth:
When someone gives you a gift, don't be ungrateful.

Don't Put All Your Eggs In One Basket:
Do not put all your resources in one possibility.

Doozy:
Something outstanding.

Down To The Wire:
Something that ends at the last minute or last few seconds.

Drastic Times Call For Drastic Measures:
When you are extremely desperate you need to take extremely desperate actions.

Drink like a fish:
To drink very heavily.

Drive someone up the wall:
To irritate and/or annoy very much.

Dropping Like Flies:
A large number of people either falling ill or dying.

Dry Run:
Rehearsal.