Based on the stoner term weaksauce, nubsauce is directedly more specifically at a person who completely fails at life. Usually tossed around in online circles, it denotes somebody who is not just a noob, but is completely lame as well.
An idiot online who has been around for ages, and still doesn't know how the hell to do things in the video games he plays. Nubsauce is often used together with l2play.
Halo's been out for forever, and he still plays it, and he still sucks at it. What a nubsauce.