"shooting the boot" is something done in rugby, during post match drink-ups either at the bar, party, wherever.
When group sing-a-longs happen, and if you were the person leading the song, and mess up the lyrics you have to shoot the boot...This means take the nastiest rugby boot (cleat/ shoe), and fill it with whatever alcohol was lying around (and sometimes spit, and other stuff).
Some of the older rugby songs have some of the nastiest lyrics by the way. I think the whole practice has become a bit tamer over the years.