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The basis of this name would appear to be kos which means "blackbird" in Czech, Polish and other Slavic languages. Kosek, Kosik and Kosicki are all Polish surnames, Kosik is also a Czech surname. Kosicki could be from a place name, the others have diminutive endings.
On blackbirds, Hanks and Hodges, in the "Oxford Names Companion", have this to say; "Blackbirds had a reputation for being cunning or devious, so the nickname would have been used to refer to a smart or unscrupulous person."