Fetching details from the dictionary...
trembling (with fear, cold, etc.); shaking; chattering (of teeth)
coming loose; wobbling
膝ががくがく震えている。
My knees are trembling
寒くて歯ががくがく鳴る。
My teeth are chattering from the cold
緊張で足ががくがくした。
My legs were shaking from nervousness