This is a common idiom used to respond to something we find unbelievable. For example, if a couch potato tells you that he or she completed a marathon, you might respond by saying: ‘Are you pulling my leg?’ because you think the situation is impossible. In addition, if you pull someone’s leg, this means you are trying to make someone believe something is true when it isn’t. We usually do this as a joke, i.e. in situations that aren’t very serious.
OMG Janette, I still don’t get it!! OK, OK, check out this definition.