An expression of satisfaction, usually said in an abnormally deep voice, regardless of whether the speaker is male, female or otherwise.
Also used to express moments of total awesomeness.
Example: "Hey, guess what? Frank stuck a bottle cap up his nose and had to go to the emergency room to get it removed."
Your Response: "Haha, dece."
For a greater effect, it may be elongated (while simultaneously using a deep voice).
Example: "I got us tickets to see the Stones, man!"
Your Response: "Oh, deeeeeece!"
(Please note that an abnormally high pitched voice may also be used, within certain circumstances, in replacement of the lower pitched voice. The preceding example is one of these instances.)
ORIGIN: Slang; a shortening (and distortion) of the word 'decent'.