It means cake eater which means being spoiled. However this term is not used the same way as the Marie Antoinette 'let them eat cake' saying which referrs mainly to her wealth and unawareness.
Keksa is used to show laziness, weakness and being spoiled from not having to work for what you have 'earned'.
If you hear a Croatian call you this, now you know what they are saying! LOL
Mostly used in the eastern part of the US, i.e., Pennsylvania.