Someone or something not worth knowing. A social pest and/or idiot. When something annoys you, its a jam tin.
That annoying crowe is such a jam tin.
A type of improvised grenade-like bomb used mainly in the first world war. Soldiers would use the empty tins from their ration of jam and fill it with explosives and shrapnel. Improvised jam tins often resemble the 'Double Cylinder Bomb' that was in use at the same time.
"I managed to kill two guys in trenches today using a couple of jam tins"
"Shit, a jam tin went off next to me and now I've got shrapnel stuck in my leg"