1. A greeting. Called out in a high pitched voice similar to that used in Worms games.
2. A warcry. Shouted in the same high pitched tones. Not scary at all to the enemy, but often used as an understatement to lull them into a false sense of security (called out gently before launching a huge missile, for example.) This use also originates from the Worms default voice.
1. Hello. Fadorkin!
2. Fadorkin! (BOOM)
One note - do not try to replicate this word, because you will invariably make a fool of yourself. I am the only one with power over this expression. Some may come close, but none can use this word correctly. There is a third use, but I am not telling it to you, arsehole.