A 'bomb' that has exploded, and lets out lots of old people.
Originated from a person who drew an old person, with a feather in her hair...
The feather turned out to look like a bomb.
A granny, with a bomb = Granny Bomb
Jaz: Come on! Lets get on that yellow and blue bus, we're late.
Kajol:Ok quick its going to go...
Alex: EW NO !!!! That's had a granny bomb on it.