When a player sits at their own spawn not helping their own team at all (almost). This is usually in FPS games to camp and snipe or to look AFK and then kill an enemy this is mostly in Call Of Duty and sometimes in Overwatch. in Overwatch the spawn sitters might be bastion, symetra, torbjorn or any offence / defence character. In call of duty... just don't got to their spawn... its that simple and it doesnt matter what they're using unless they're a sniper demolishing your team.
"Lets spawn sit" or "they're spawn sitting!" or ever "he's a spawn sitter"