Abandon computers? You must be joking! (4,3,2)
I believe the answer is:
come off it
'you must be joking' is the definition.
(both sayings can be used to express disbelief)
'abandon computers?' is the wordplay.
'abandon' becomes 'come off' (as in coming off drugs).
'computers?' becomes 'IT' (information technology).
'comeoff'+'it'='COME OFF IT!'
(Other definitions for come off it that I've seen before include "stop behaving like that!" , "Don't be ridiculous!" , "Don't make me laugh" , "Don't talk rubbish" , "Stop messing about!" .)