ok, I know orks very well for some reasons (I think they're cool, and my friend plays them a lot). And more dakka, and dakka in general refers to the sound that a machine gun makes when shot:
DAKKA DAKKA DAKKA
you know, like shooting, and stuff. It's thrown around a lot to mean shooting, killing, shooting a lot (the ork upgrade gives weapons an extra shot), and that sound
WAAAAGH!!!!