Although the adjective Scotch is used by Shakespeare - a "Scotch jig" is referred to in Much Ado About Nothing (II.i.65) - the use of the word on its own to refer to a drink of whisky is much later, whisky itself being an 18th-century innovation. So when Scarus boasts to Antony that he has "Room for six scotches more" (Antony and Cleopatra, IV.vii.10) we must try not to let the modern sense interfere. Scarus is showing bravery: the word means "cut" or "gash". We hear it used as a verb in Coriolanus, when one servant tells another of how Coriolanus fought Aufidius: "Before Corioles he scotched him and notched him" (IV.v.193). The word is very close in meaning to scorch "cut with a knife", used when Macbeth tells his wife "We have scorched the snake, not killed it" (Macbeth, III.ii.13), and some editors have (unnecessarily) replaced the word by scotched.
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin