A word/adjective; to descibe something that is powerful/joyful/dramatic.
Normally shouted out loud.
When Wayne Rooney scores a screamer you would shout 'boomshanks! what a goal that was.'
Or when you've just won a bet in the bookies you again would shout 'Boomshanks!'
When a boxer gets knocked out you would shout 'Boomshanks, take that!'