Why can't wemmick in Great Expectations say magwitch's name?

Answer 1

Magwitch/Provis's name is initially unknown to Pip; he in fact believes his benefactor to be Miss Havisham. Neither Jaggers nor Wemmick can legally or ethically mention it, based on the conditions of the agreement they are executing by funding Pip. Late in the novel, Magwitch/Provis is a fugitive and to name him might lead to his capture.

Answer 2

Wemmick is afraid of speaking Magwitch's name because he is cautious about revealing sensitive information that could put Pip and himself in danger. Additionally, Wemmick's professional identity as a law clerk means he must be discreet about certain matters, especially those related to his clients.