means A small, roughly built hut or cabin, typically flimsy and barely furnished.

from American English of the late 19th century, origin debated. One popular suggestion ties it to Mexican Spanish 'jacal' (from Nahuatl 'xacalli,' an adobe hut), the sound worn down into 'shack.' Others link it to the dialectal English verb 'shackly,' meaning rickety or shaky — which fits a building that wobbles. The honest answer is that nobody is certain, though both stories suit a structure held up mostly by optimism.