From Longman Dictionary of Contemporary Englishno kiddingno kiddingspoken a) used to say that you understand and agree with what someone has just said ‘That girl has some major problems.’ ‘Yeah, no kidding.’ b) used to emphasize a threat or that you are telling the truth If you break that thing, you’ll be grounded for a week – no kidding. And then he saw us and – no kidding – he asked us if we wanted a ride. → kid
Examples from the Corpusno kidding• "Man, physics class is hard!" "No kidding!"• "She's getting married again." "No kidding?"• I'm telling you, this guy's as fast as Carl Lewis -- no kidding!• If you want to get a slice of the action book early - no kidding.• No kidding, how about that!• No kidding, it seems like they always come in pairs-for Patty, I mean.• For a while, there was a federal lawsuit being filed even day... no kidding, one a day.• Me: No kidding, since when?• No kidding? You mean Becky's actually going to Princeton?