The incident occurs in Chapter 2, when Pip is trying to gather together food to give to the convict who threatened him in the graveyard. It was considered unhealthy and dangerous to ‘bolt’ one’s food at this time, so Mrs Joe administers a medicine to Pip which is made of tar water-
Some medical beast had revived Tar-water in those days as a fine medicine, and Mrs Joe always kept a supply of it in the cupboard; having a belief in its virtues correspondent to its nastiness.
She appears, however, to use the tar water as much as a punishment as a cure. The fact that she makes Joe take the medicine too, though he was eating carefully throughout, shows there is more cruelty than benevolence in her actions.