A fail way to say "underestimate", but most commonly used in the Russian community of underground (Basement dwellers) fat kids who where make-up and pretend to hack the interwebz. But at the same time look for posts via forums and try to find a problem they can take credit for without any valid proof showing that they are the ones responsible. Thus attracting the troll squad who flames them endlessly and pisses them off to the point where they either flee from the underground and seek a new life elsewhere.... or sit and try to fight an endless wave of trolls.........
However some are still trying to learn English so they post big words to sound good, but are not aware those "big words" are improperly typed. (Sebastimate, Sebestimate)
vpxsyngexvpx: ok No Problem. Us Servers Is Going Down Again Now. Cuz You Sebestimate Me.
Troll: lol Sebastimate
Troll2:srsly learn some grammar, and ask the real attackers to show up, we don't want you here anymore.
Troll3: Sebestimate me baby!! Sebestimate me all over!!!!
Troll4: The mods just have too much fun lolling at you sebestimating eachother...