( informal ) used to say that you do not believe sth that sb has just told you :
'I'm trying as hard as I can!' 'You could have fooled me!'
( informal ) used to say that you do not believe sth that sb has just told you :
'I'm trying as hard as I can!' 'You could have fooled me!'
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005