Tvrd je orah voćka čudnovata, ne slomi ga, al’ zube polomi.
A hard nut is a strange fruit, you won't break it, but you'll break your teeth.
By Petar II Petrović-Njegoš
This is a line from his epic poem "The Mountain Wreath" and is part of a reply to a vizier Selim who requested unconditional surrender from "tiny" Montenegro which he insulted for being small. The response is saying that although Montenegro is tiny, it will be a tough nut to crack if Ottomans try to conquer it. Nowadays, it means one should not underestimate others, no matter how small/weak they seem.