More shaky or in worse condition than something else.
The word 'shackier' makes people think of things that are not very strong or safe.
Examples:
The old house is shackier than the new one.
Her bike is shackier than mine.
The shackier chair broke when I sat on it.
My little brother's tower of blocks is shackier than mine.
The shackier the bridge, the scarier it is to cross.
The shackier the boat, the more fun it is to ride in.