I can only offer a guess, but I'd guess that the original name was Kubacki. This would have been pronounced: K as in kentucky U as in the oo in cool B as in Bad A as in father C like the ts in Tsar K as in Kentucky I as in the word eye.

So, KUBATSKI.... But since it does not look like that to non-Polish eyes, I'd guess Koboski was an anglicization of the name Kubacki. I think that as Poles tried to assimilate they tried to make their names stand out less to other Americans. Of course, this is only a guess, but I believe it's a reasonable guess. I've seen this happen a number of times over the years to Polish names.