jeshiva bucher

a young man

skinny or ill-looking

The Jeshiva Bucherim were the apprentices in rabbinic schools at the times of Tsarist Russia (there were many such schools from Lithuania to Ukraine); the training was said to be very hard and the students would eat only salted bread, hence the say: "poor guy, he looks like a Jeshiva Bucher" for a skinny boy.