In UK slang, to "mong out" is to relax on a sofa in a state of immobility, generally after overenthusiastic herbal indulgence.

It may be related to the insult against those with downs syndrome, but people aren't thinking about that when they say it. They're thinking about lying on their bed with a laptop having just woken up at lunchtime, browsing through funny pictures using only the arrow keys.