To easily dominate new, less-skilled, or inexperienced players. There are many games in which one can 'Nubstomp', all being multi-player PvP. Such games as Call of Duty ?, DotA 1-2, League of Legends, ect.
The prefix "Nub" is a variation of "noob" which derived from "newbie".
The word "Nubstomp" is a variation of "Pubstomp".
"dude, master yi is so fun."
"yeah, hes always been easy to nubstomp with"