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The title as written is a regional mispronunciation; it should be Koca Dünya. A Turkish expert writes: "K <-> G happens, not only in rural (esp. eastern) Turkish but also in Persian where they often pronounce ق like g rather than q as in Arabic."
On the meaning of that first word, our expert writes:
Koca can mean "large," or it can mean "old," depending on the context. I see in the lyrics that they played it safe and wrote "big old world." I think that in the song it is more likely to mean "old", as in "hey, old friend, how are you?"
Lyrics (in Turkish), with translation