I have always heard the phrase in the form, ‘dressed fit to kill’; it may be a coincidence, but I find a nearly identical phrase (in Greek) in Herodotus’ Histories, Book I ch. 109: There the child Cyrus is delivered by his grandfather, the Median King Astyages, to his steward Harpagus to be killed by exposure or otherwise, ‘kekosmhmenon thn epi thanatwi’, ‘dressed up [in clothes & ornaments] for death’. I think the phrase may have originally meant something like, ‘dressed up as you would be for your own funeral’, especially as in this case, a royal one.