his grief only became more deep and rankling when he had leisure for reflection, and at length it took so fast hold of his mind that at the end of three months he lay on a bed of sickness
What does "hold" mean here?
I don't know if I got it right but I understand that as the capacity of the mind to bear the situation
The word "hold" is part of the phrasal verb "took hold", which is divided here by an adverb "so fast". The pronoun "it", the thing that it taking hold of his mind, refers to "[h]is grief", the subject of the previous clause.
This passage might be rephrased in easier English as follows:
His grief only became more deep and rankling when he was free to think. At length, his grief dominated his mind so much that after three months he lay on a sickbed.
