We've got a serious emergency on our hands: Tristram is black in the face and the parson is just waiting for Mr. Shandy to give him a name so he can baptize him before death. But Mr. Shandy isn't dressed. He gives the name to Susannah, who's run to find him.
He's afraid, though, that flibbertigibbet Susannah isn't going to be able to keep the name Trismegistus in her head the whole way down the hall.
Good call, Mr. Shandy.
She remembers the first part, Tris, and the parson figures that Mr. Shandy must mean Tristram. The parson baptizes him, assures Mr. Shandy, who's just arrived, that he got the name right (although without telling him what that name is), and Mr. Shandy heads back to get fully dressed.