A popular onomatopoeia on IRC and other text-based chatting protocols. It can be used to represent success or excitation in any arena, but more often is used in a sexual connotation (hint: say it out loud a few times and think techno beat). This term is most frequently employed by members of the male gender, though it is not unheard of for a female chatter to employ it.
Here we have an example of how the term might be used in the wild:
--> You are now talking on #1337d00d5
<31337h4XX0r> y0, i t0t4lly fuxx0red this bizatch l4st nite, y0
<|<illa|<ris> h4h4h4h4h4h4 unts
<5criptbitch> did you put it all the way in, yo?
<|<illa|<ris> yah, did you fuxxor her hardc0re?
<31337h4XX0r> :) unts unts unts unts
<5criptbitch> hahahaha, unts unts unts fap fap
(see also: untz)