So slim and skinny/weak are synonymous in Romanian? Slim can be a compliment though skinny/weak less so! :)
For polyglots out there: is there anything that you can associate with the word "slab"?
Perhaps "weak" would be the most suitable translation? In Russian, слабый (slabyi) means literally "weak", and these words do have a similarity. Nevertheless, I'm not a Romanian speaker...
In my head i think of a slab of marble as like a thin piece used for a counter or a slab of rock as like a thin section of rock coverinh something. Not like paper thin, but I dont think it means that here either. Hope that helps :)