verb. When an organized team of friends/clan members plays against a team of "pubs" - random people who join a public server.
The game is typically very one sided, despite the games being created under the false impression that it is going to be 'fair'.
Can apply to any team-oriented game, but the term is likely to have originated in DotA.
Can also be used as a noun, whereby the events described above occurs.
"Lets go pubstomp some noobs. I'll host"
"5v5 noobs only. nothing suss"