No kidding
Definition 1 of 2
Pro Tip 1/3Confirming Obviousness
Use 'no kidding' to reinforce something that is clearly true or apparent to everyone. 
It's hot outside in July. No kidding, we're in Texas.
Pro Tip 2/3Sarcasm Indicator
'No kidding' can indicate sarcasm when responding to obvious statements or questions. 
We lost the game 10-0. No kidding, it was a disaster.
Pro Tip 3/3Informal Agreement
'No kidding' is an informal way to express vehement agreement with what has just been mentioned. 
This book is really boring. No kidding, I couldn't even finish it.