Being a "Sm" is basically being awful at every game you play. This also includes swearing extensively while dying in a FPS. This is named after a particularly bad player at Halo 2, Halo 3, and Call of Duty 4.
Until the IRA declared a ceasefire SM was said by thousands of people travelling from Belfast City Airport, Belfast International Airport, the ferries sailing from Larne or Belfast and all the people entering Belfast city centre through the security gates .