Define Down B Meaning

To give someone a sweet uppercut (in the style of Mortal Kombat.)

Upside down b - which if you look at it as a lower case, means p, or methamphetamine. This is a common term used in New Zealand, made famous by Bro'Town.
The letter p is likely to be derived from the main ingredient in methamphetamine, pseudoephedrine.

Someone or something thats retarded.
This is an alternative word to retarded.