D for for abbreviating the word Dampener.
A person who does a 'D' is someone whom starts a
flame and before the
flame can grow into a bonfire, the person dampens the
mood just at the precise point of peaking.
Denise said she was going to introduce a
girl to her buddy and has been raving about how
good this female friend of hers is. Just as her buddy took the
bait and decide to go for it, Denise decided do a 'D' and asked her buddy to take a chill pill instead.