why 'up' and not 'off' here? I love the examples supplied in the book as they are given with the foreign student in mind and they are You might know as many words as you can count in the stars in the sky, but if you know not what phrasal verbs are, it availeth not. & You can say something like that but you have nothing to back it UP. These are the verbs that would enable you to tell the difference between: Back off, buddy! He backed off on us in the last minute. You might know as many words as you can count in the stars in the sky, but if you know not what phrasal verbs are, it availeth not.
