'd take somebody mighty used to the dark to make a competent witness . . .’
This
shows how Mr Tate is trying to hint
at how Boo Radley was there when Mr Ewell died. He says that Boo Radley had spent a long time
indoors. Atticus misunderstood what he
was saying and thought that it was an attempt to shift the blame from Jem to Boo
Radley.