An expression of appreciation or an exclamation of enthusiasm for something. Plays on the idea of the joy one gets from delivering a fatal gunshot wound to a foe. Has it's origins from Jamaican immigrants to Great Britain who use it as a sound byte when performing as a vocalist over neo-modern quazi-urban musical stylings. Similar to bumbaclot, perhaps has the opposite intended meaning.
I was dancing in the clubbing establishment with my close aquaintances, when a most agreeable bass line was dropped by the record jockey. I did exclaim 'Boom Fatal, that sound is off the proverbial chain!'