Pronounced mun-j or munje, when you mundge about you keep doing lots of slightly entertaining things for a while until you get bored after a minute or so, then do something else, eg. play one level of a game, watch a short youtube video, generally walk about for a bit thinking what to do next, listen to a piece of music, watch a bit of TV etc. Mundging around is usually used to find something entertaining enough you don't get bored, like mundging about until you go on YouTube and realise it is entertaining enough to watch more and stop mundging.
"Yesterday I was just in a mood where there was nothing to do but mundge around. I finally found playing TF2 entertaining enough to waste the rest of the day"