Can you explain the expression a drop of scotch?

Scotch is type of whiskey (an alcoholic beverage).

The phrase about "a drop of scotch" would be used by someone trying to be cute or coy about their desire to drink some scotch. They are saying (usually insincerely) that they will have a little bit of scotch to drink. It is a convivial phrase for a drink of scotch, usually encountered in an invitation: Will you have a drop of scotch with me? Or it may indicate a small amount added to something: I'll have water with a drop of scotch.