This is distinct from the imperative "Fuck off!" You try to say it as one word with the emphasis on the "fuck". This word has a couple of uses in Australia:
1. To qualify an adjective, if something is REALLY big or REALLY brightly coloured, for example, then you slide "fuck-off" in somewhere. This is also used in the UK and is seen in the movie Lock Stock and Two Smoking Barrels: "Also, I think knives are a good idea. Big, fuck-off shiny ones."
2. Another term for insect repellent.
1. My sister's new car is bright fuck-off yellow.
2. These fucking flies are shitting me. Anyone got some fuck-off?